More on SQL. Juliana Freire. Some slides adapted from J. Ullman, L. Delcambre, R. Ramakrishnan, G. Lindstrom and Silberschatz, Korth and Sudarshan
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1 More on SQL Some slides adapted from J. Ullman, L. Delcambre, R. Ramakrishnan, G. Lindstrom and Silberschatz, Korth and Sudarshan
2 SELECT A1, A2,, Am FROM R1, R2,, Rn WHERE C1, C2,, Ck Interpreting a Query Translate to Relational Algebra 1. Start with the Cartesian product of all the relations in the FROM clause. 2. Apply the selection condition from the WHERE clause. 3. Project onto the list of attributes and expressions in the SELECT clause.
3 Interpreting a Query SELECT A1, A2,, Am FROM R1, R2,, Rn WHERE C1, C2,, Ck Nested loops Imagine one tuple-variable for each relation in the FROM clause. These tuple-variables visit each combination of tuples, one from each relation. If the tuple-variables are pointing to tuples that satisfy the WHERE clause, send these tuples to the SELECT clause.
4 Challenge Question Suppose R, S and T are unary relations, each having one attribute A. We want to compute R (S T). Does the following query do the job? SELECT R.A FROM R, S, T WHERE R.A = S.A OR R.A = T.A???
5 SELECT FROM WHERE ORDER BY GROUP BY HAVING SQL Extensions Extension to the SELECT clause e.g., DISTINCT, SUM, COUNT, MIN, MAX, AVG and AS Extension to the FROM clause e.g., correlation names and various kinds of JOINs Extension to the WHERE clause e.g., AND, OR, NOT, comparators, EXISTS, IN, ANY Several additional clauses e.g., ORDER BY, GROUP BY, and HAVING (SELECT FROM WHERE ) UNION (SELECT FROM WHERE ) And operators that expect two or more complete SQL queries as operands e.g., UNION and INTERSECT
6 Sample Database For this discussion, we will use the following database: Customer (Number, Name, Address, CRating, CAmount, CBalance, RegisterDate, SalespersonNum) Foreign key: Customer.SalespersonNum references Salesperson.Number Salesperson(Number, Name, Address, Office)
7 Eliminating Duplicates Consider the following two queries: Name W. Wei SELECT DISTINCT Name FROM Customer; SELECT Name FROM Customer; J. Smith Name J. Smith W. Wei J. Smith The first query eliminates duplicate rows from the answer. Although the relational model is based on set, by default RDBMSs operate on multisets (bags) The query writer gets to choose whether duplicates are eliminated
8 Eliminating Duplicates: A Word of Caution In theory, placing a DISTINCT after select is harmless In practice, it is very expensive The time it takes to sort a relation so that duplicates are eliminated can be greater than the time to execute the query itself! Use DISTINCT only when you really need it
9 Aggregates Summarize or aggregate the values in a column Operators: COUNT, SUM, MIN, MAX, and AVG Apply to sets or bags of atomic values SUM and AVG: produce sum and average of a column with numerical values MIN and MAX: applied to column with numerical values, produces the smallest and largest value applied to column with character string values, produces the lexicographically first or last value COUNT: produces the number of values in a column Equivalently the number of tuples in a relation, including duplicates SELECT AVG (CBalance) How is this query evaluated? FROM Customer WHERE age > 35;
10 Aggregates and NULLs General rule: aggregates ignore NULL values Avg(1,2,3,NULL,4) = Avg(1,2,3,4) Count(1,2,3,NULL,4) = Count(1,2,3,4) But Count(*) returns the total number of tuples, regardless whether they contain NULLs or not
11 Aggregates and Duplicates Aggregates apply to bags If you want sets instead, use DISTINCT SELECT COUNT(Name) SELECT COUNT(DISTINCT Name) FROM Customer; FROM Customer; Answer: 3 Answer: 2 Name J. Smith W. Wei J. Smith
12 Note: Full-Relation Operations DISTINCT and aggregates act on relations as a whole, rather than on individual tuples More on aggregates later!
13 SQL Extensions SELECT FROM WHERE Extension to the SELECT clause e.g., DISTINCT, SUM, COUNT, MIN, MAX, AVG and AS Extension to the FROM clause e.g., correlation names, subqueries and various kinds of JOINs
14 Joins There are a number of join types that can be expressed in the FROM clause: inner join (the theta join) cross join (Cartesian product) natural join left outer join right outer join full outer join union join
15 Joins There are a number of join types that can be expressed in the FROM clause: inner join (the theta join) cross join natural join left outer join right outer join full outer join union join These are syntactic sugar they can be expressed in a basic SELECT..FROM..WHERE query.
16 Joins There are a number of join types that can be expressed in the FROM clause: inner join (the regular join) cross join natural join left outer join right outer join full outer join union join There are new operators but can be expressed in a complex SQL query involving the union operator.
17 ON clause for the join Join condition in the ON clause (vs. the WHERE clause) These two queries are equivalent: SELECT C.Name, S.Name FROM Customer C JOIN Salesperson S ON C.SalespersonNum = S.Number WHERE C.CRating < 6; SELECT C.Name, S.Name FROM Customer C, Salesperson S WHERE C.SalespersonNum = S.Number AND C.CRating < 6; Customer (Number, Name, Address, CRating, CAmount, CBalance, RegisterDate, SalespersonNum) Salesperson(Number, Name, Address, Office)
18 Basic Join INNER JOIN These queries are equivalent. Customer (Number, Name, Address, CRating, CAmount, CBalance, RegisterDate, SalespersonNum) Salesperson(Number, Name, Address, Office) SELECT C.Name, S.Name FROM Customer C JOIN Salesperson S ON SalespersonNum; SELECT C.Name, S.Name FROM Customer C INNER JOIN Salesperson S ON SalespersonNum; Note the use of foreign key to simplify expression For the INNER JOIN, the query answer does not include: a Customer that doesn t have a Salesperson. or a Salesperson that is not assigned to any customers.
19 Equijoin and Natural Join Equijoin: Join condition has only equality Result will contain two attributes with identical values, perhaps different names Natural join: Equijoin on attributes with same names No need to specify join attributes Result will contain each join attribute only once What if there are no common attribute names? In that case, Natural Join Cross Product
20 Natural Join Joins attributes with same name and eliminates one of them from the result SELECT * FROM Customer NATURAL JOIN SalesPerson; How can we write an equivalent query without using a join clause? Customer (Number, Name, Address, CRating, CAmount, CBalance, SalespersonNum) Salesperson (Number, Name, Address, Office)
21 Original query: Natural Join SELECT * FROM Customer C NATURAL JOIN SalesPerson S; Equivalent query: SELECT C.Number, C.Name, C.Address, C.CRating, C.CAmount, C.CBalance, C.SalespersonNum, S.Office FROM Customer C, Salesperson S WHERE C.Number = S.Number and C.Name = S.Name, and C.Address = S.Address; Customer (Number, Name, Address, CRating, CAmount, CBalance, SalespersonNum) Salesperson (Number, Name, Address, Office) Project out repeated columns BTW, what does this query compute?
22 Natural Join: Some Notes equivalent SELECT * FROM Customer C, Salesperson S WHERE C.SalespersonNum = S.Number; SELECT * FROM Customer C JOIN Salesperson S on SalespersonNum; SELECT * FROM Customer NATURAL JOIN Salesperson this query is not equivalent to above two queries, why? Customer (Number, Name, Address, CRating, CAmount, CBalance, SalespersonNum) Salesperson (Number, Name, Address, Office)
23 How would you write the following query? Student(sid, name, address) Spouse(sid, name), sid references Student.sid List the names of all students and their spouses, if they have one. SELECT Student.name,Spouse.name FROM Student, Spouse WHERE Student.sid=Spouse.sid Does this SQL query do the job?
24 How would you write the following query? Student(sid, name, address) Spouse(sid, name), sid references Student.sid List the names of all students and their spouses, if they have one. SELECT Student.name,Spouse.name FROM Student, Spouse WHERE Student.sid=Spouse.sid Does this SQL query do the job? No! Students without spouses will *not* be listed.
25 Outer Join An extension of the join operation that avoids loss of information. Computes the join and then adds tuples from one relation that do not match tuples in the other relation to the result of the join. Uses null values to pad dangling tuples
26 LEFT OUTER JOIN Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 INNER JOIN on C.SalespersonNum = S.Number gives us: smith with johnson and jones with johnson LEFT OUTER JOIN on C.SalespersonNum = S.Number gives us: INNER JOIN plus wei with <null> salesperson - Lists all customers, and their salesperson if any
27 RIGHT OUTER JOIN Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 INNER JOIN on C.SalespersonNum = S.Number gives us: smith with johnson and jones with johnson RIGHT OUTER JOIN on C.SalespersonNum = S.Number gives us: INNER JOIN plus <null> customer with miller - Lists customers that have a salesperson, and salespersons that do not have a customer
28 FULL OUTER JOIN FULL OUTER JOIN = LEFT OUTER JOIN RIGHT OUTER JOIN FULL OUTER JOIN on C.SalespersonNum = S.Number gives us: INNER JOIN plus wei with <null> salesperson plus <null> customer with miller - Lists all customer-salesperson pairs, and customers that do not have a salesperson, and salespersons that do not have a customer
29 CROSS JOIN A CROSS JOIN is simply a cross product SELECT * FROM Customer CROSS JOIN Salesperson; How would you write this query without the CROSS JOIN operator? SELECT * FROM Customer, Salesperson;
30 SQL Extensions SELECT FROM WHERE Extension to the SELECT clause e.g., SUM, COUNT, MIN, MAX, AVG and AS Extension to the FROM clause e.g., correlation names and various kinds of JOINs Extension to the WHERE clause e.g., AND, OR, NOT, comparators, EXISTS, IN, ANY
31 WHERE Clause: Comparison Operators <, >, =, <>, >=, <= Compare two values as expected Operates on numbers as well as text values Amount < 50 and CustID <> 1 custname = Juliana Freire (string concatenation) LIKE Compare a text value with a pattern % compares with zero or more characters _ compares with exactly one character custname LIKE %Fr re matches Juliana Freire, Freire, Friere
32 Subqueries A parenthesized SELECT-FROM-WHERE statement (subquery ) can be used as a value in a number of places, including FROM and WHERE clauses. Example: in place of a relation in the FROM clause, we can use a subquery and then query its result. Must use a tuple-variable to name tuples of the result.
33 Subqueries in the FROM clause The FROM clause takes a relation, but results from SQL queries are themselves relations, so we can use them in the FROM clause, too! SELECT (N.CRating+1) AS CIncrRating FROM (SELECT * FROM Customer WHERE CRating = 0) AS N WHERE N.CBalance = 0 This can often be a more elegant way to write a query, but will be slower. Why? Can this be written without nesting? SELECT (CRating+1) AS CIncrRating FROM Customer WHERE CRating = 0 AND CBalance = 0
34 SELECT FROM Subqueries in the WHERE clause C1.Number, C1.Name Customer C1 This is a complete query WHERE C1.CRating IN (SELECT MAX (C2.CRating) FROM Customer C2;); Find all customers where their credit rating is equal to the highest credit rating that appears in the database. To understand semantics of nested queries, think of a nested loops evaluation: for each customer tuple, check the qualification by computing the subquery
35 IN <subquery>: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating IN (SELECT MAX (C2.CRating) FROM Customer C2;); SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating IN {10} Result: 3, wei
36 NOT IN <subquery>: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating NOT IN (SELECT MAX (C2.CRating) FROM Customer C2;); SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating NOT IN {10} Result:?
37 Conditions Involving Relations: IN and NOT IN SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating IN (SELECT MAX (C2.CRating) FROM Customer C2;); <attribute-name A> IN (subquery S): tests set membership A is equal to one of the values in S <attribute-name A> NOT IN (subquery S) A is equal to no value in S
38 Conditions Involving Relations: EXISTS EXISTS R is true if R is not empty NOT EXISTS R is true if R is empty SELECT C.Name FROM Customer C WHERE EXISTS (SELECT * FROM Salesperson S WHERE S.Number = C.SalespersonNum;); If the answer to the subquery is not empty - then the EXISTS predicate returns TRUE Tests for empty relations What does this query compute?
39 EXISTS: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 SELECT C.Name FROM Customer C WHERE EXISTS (SELECT * FROM Salesperson S WHERE S.Number = C.SalespersonNum;);
40 Conditions involving relations: NOT EXISTS NOT EXISTS R is true if R is empty SELECT C.Name FROM Customer C WHERE NOT EXISTS (SELECT * What does this query compute? FROM Salesperson S WHERE S.Number = C.SalespersonNum;); If the answer to the subquery is empty - then the NOT EXISTS predicate returns TRUE Tests for non-empty relations
41 NOT EXISTS: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 SELECT C.Name FROM Customer C WHERE NOT EXISTS (SELECT * FROM Salesperson S WHERE S.Number = C.SalespersonNum;);
42 Set comparison: ALL or ANY in a Subquery Syntax: attribute-name comparator ALL (subquery) attribute-name comparator ANY (subquery) A > ALL (subquery S): True if A is greater than every value returned by S (A <> ALL S) (A NOT IN S) A > ANY (subquery S) True if A is greater than at least one value returned by S (A = ANY S) (A IN S)
43 ALL or ANY in a Subquery: Example This predicate must be true for all SELECT S.Number, S.Name SalespersonNums returned by FROM Salesperson S the subquery! WHERE S.Number = ALL (SELECT C.SalespersonNum FROM Customer;); What does this query compute? Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Salesperson Number Name Address Office 101 johnson aaa miller bbb 26
44 ALL or ANY in a Subquery: Example SELECT C.Name FROM Customer C WHERE C.Crating >= ALL (SELECT C1.Crating FROM Customer C1); What does this query compute? Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Salesperson Number Name Address Office 101 johnson aaa miller bbb 26
45 Correlated Subqueries The simplest subqueries can be evaluated once and for all and the result used in a higher-level query More complicated subqueries must be evaluated once for each assignment of a value to a term in the subquery that comes from a tuple outside the subquery SELECT S.Number, S.Name FROM Salesperson S WHERE S.Number IN (SELECT C.SalespersonNum FROM Customer C WHERE C.Name = S.Name;); Because the subquery mentions an attribute from a table in the outer query
46 Subquery that is not correlated SELECT C1.Number, C1.Name FROM Customer C1 WHERE C1.CRating IN (SELECT MAX (C2.CRating) FROM Customer C2;); The subquery only uses attributes from the the table mentioned in the subquery
47 Correlated Subqueries: Scoping An attribute in a subquery belongs to one of the tuple variables corresponding to the closest relation In general, an attribute in a subquery belongs to one of the tuple variables in that subquery s FROM clause If not, look at the immediately surrounding subquery, then to the one surrounding that, and so on.
48 Correlated Subqueries: Semantics Analyze from the inside out For each tuple in the outer query, evaluate the innermost subquery, and replace that with the resulting relation Repeat SELECT S.Number, S.Name FROM Salesperson S WHERE S.Number IN (SELECT C.SalespersonNum FROM Customer C WHERE C.Name = S.Name;);
49 Correlated Subqueries: Semantics SELECT S.Number, S.Name FROM Salesperson S WHERE S.Number IN (SELECT C.SalespersonNum As we range through the Salesperson tuples, each tuple provides a value for S.Name FROM WHERE Can t evaluate, don t know The value for S.Name Customer C C.Name = S.Name;); De-correlate: another way to write this query: SELECT S.Number, S.Name FROM Salesperson S, Customer C WHERE S.Number = C.SalespersonNum AND C.Name = S.Name; These two queries are equivalent. Is one preferable to the other?
50 SELECT FROM WHERE ORDER BY GROUP BY HAVING SQL Extensions Extension to the SELECT clause e.g., SUM, COUNT, MIN, MAX, AVG and AS Extension to the FROM clause e.g., correlation names and various kinds of JOINs Extension to the WHERE clause e.g., AND, OR, NOT, comparators, EXISTS, IN, ANY Several additional clauses e.g., ORDER BY, GROUP BY, and HAVING
51 ORDER BY Sort the result on one or more attributes Can specify ASC, DESC--default is ASC SELECT Name, Address FROM Customers ORDER BY Name SELECT * FROM Customer C JOIN Salesperson S ON C.SalespersonNum ORDER BY CRating DESC, C.Name, S.Name
52 ORDER BY: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 <null> Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 SELECT Name, Address FROM Customers ORDER BY Name Answer: Jones, yyy Smith, xxx Wei, zzz
53 ORDER BY: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 SELECT * FROM Customer C JOIN Salesperson S ON C.SalespersonNum ORDER BY CRating DESC, C.Name, S.Name Answer: 3, wei, zzz, 10 2, jones, yyy, 7 1, smith, xxx, 5
54 ORDER BY: Example Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, Ann aaa 7 3,000 20, wei zzz 10 10,000 10, Salesperson Number Name Address Office 101 johnson aaa miller bbb 26 SELECT * FROM Customer C JOIN Salesperson S ON C.SalespersonNum ORDER BY CRating DESC, C.Name, S.Name Answer: 4, wei, zzz, 10 3, ann, aaa, 7 2, jones, yyy, 7 1, smith, xxx, 5
55 Grouping GROUP BY partitions a relation into groups of tuples that agree on the value of one or more columns Useful when combined with aggregation apply aggregation within each group Any form of SQL query (e.g., with or without subqueries) can have the answer grouped The query result contains one output row for each group
56 GROUP BY SELECT SalespersonNum, COUNT(*) as TotCust FROM Customer GROUP BY SalespersonNum; Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Group 1 Group 2 Answer SalespersonNum TotCust
57 Challenge Question What is the answer for the query: SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, ???
58 Challenge Question What is the answer for the query: SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Answer SalespersonNum
59 Another Challenge Question Can you write a simpler SQL stmt for this query? SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum SELECT DISTINCT SalespersonNum FROM Customer Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10, Answer SalespersonNum
60 HAVING Clauses Select groups based on some aggregate property of the group E.g., Only list a salesperson if he/she has more than 10 customers The HAVING clause is a condition evaluated against each group A group participates in the query answer if it satisfies the HAVING predicate SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum HAVING Count(*) > 10;
61 GROUP BY Clauses and NULLS Aggregates ignore NULLs On the other hand, NULL is treated as an ordinary value in a grouped attribute If there are NULLs in the Salesperson column, a group will be returned for the NULL value Customer Number Name Address CRating CAmount CBalance SalespersonNum 1 smith xxx 5 1,000 1, jones yyy 7 5,000 4, wei zzz 10 10,000 10,000 NULL SELECT SalespersonNum, Count(*) AS T FROM Customer GROUP BY SalespersonNum Answer SalespersonNum T NULL
62 GROUP BY, HAVING: Note The only attributes that can appear in a grouped query answer are aggregate operators (that are applied to the group) or the grouping attribute(s) SELECT SalespersonNum, COUNT(*) FROM Customer GROUP BY SalespersonNum; SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum HAVING Count(*) > 10; Incorrect! SELECT SalespersonNum, C.Name, COUNT(*) FROM Customer C GROUP BY SalespersonNum;
63 Readable SQL Queries SELECT SalespersonNum FROM Customer GROUP BY SalespersonNum HAVING Count(*) > 10 ORDER BY SalespersonNum Offer visual clues to the structure of query Each important keyword starts a new line Capitalize keywords Keep it compact If query or subquery is short, write in a single line SELECT * FROM Customer
64 Order of Clauses in SQL Queries SELECT FROM WHERE GROUP BY HAVING ORDER BY Required SELECT and FROM are required Can t use HAVING without GROUP BY Whichever additional clauses appear must be in the order listed
65 SELECT FROM SQL Extensions Extension to the SELECT clause e.g., SUM, COUNT, MIN, MAX, AVG and AS Extension to the FROM clause e.g., correlation names and various kinds of JOINs WHERE Extension to the WHERE clause e.g., AND, OR, NOT, comparators, EXISTS, IN, ANY ORDER BY GROUP BY HAVING Several additional clauses e.g., ORDER BY, GROUP BY, and HAVING (SELECT FROM WHERE ) UNION (SELECT FROM WHERE ) And operators that expect two or more complete SQL queries as operands e.g., UNION, INTERSECT, MINUS
66 UNIONing Subqueries (SELECT C.Name FROM Customer C WHERE C.Name LIKE B% ) UNION (SELECT S.Name FROM Salesperson S WHERE S.Name LIKE B% ); Two complete queries - with UNION operator in between. Unlike other operations, UNION eliminates duplicates!
67 UNION ALL (SELECT C.Name FROM Customer C WHERE C.Name LIKE B% ) UNION ALL (SELECT S.Name FROM Salesperson S WHERE S.Name LIKE B% ); UNION ALL preserves duplicates
68 EXCEPT (=difference) (SELECT S.Number FROM Salesperson) EXCEPT (SELECT C.SalespersonNum Number FROM Customer C); EXCEPT ALL retains duplicates What is this query looking for? Two complete queries - with EXCEPT operator in between.
69 EXCEPT (=difference) (SELECT S.Number FROM Salesperson;) MINUS (SELECT C.SalespersonNum Number FROM Customer C;); Oracle provides a MINUS operator to represent difference!
70 INTERSECT (SELECT S.Name FROM Salesperson) INTERSECT (SELECT C.Name FROM Customer C); Two complete queries - with INTERSECT operator in between. INTERSECT ALL retains duplicates What is this query looking for?
71 Bag Semantics Although the SELECT-FROM-WHERE statement uses bag semantics, the default for union, intersection, and difference is set semantics That is, duplicates are eliminated as the operation is applied.
72 Motivation: Efficiency When doing projection, it is easier to avoid eliminating duplicates Just work tuple-at-a-time. For intersection or difference, it is most efficient to sort the relations first. At that point you may as well eliminate the duplicates anyway. What about union?
73 The WITH Clause Complex queries are easier to write if you break them up into smaller components You can name a query component using the WITH clause It creates a temporary view, which is valid *only* in the query where it is defined WITH max_balance(value) AS SELECT MAX(balance) FROM Customer SELECT Cname FROM Customer C, max_balance WHERE C.balance = max_balance.value
74 Modifying the Database
75 Database Modifications Some SQL statements do not return any results They change the state of the database Insert tuples into a relation Delete certain tuples from a relation Update values of certain components of existing tuples
76 Example
77 Deletion DELETE FROM R WHERE <condition> Delete whole tuples, one relation at a time Finds and deletes all tuples t in R such that condition(t) is true Examples: Delete all account records at the Perryridge branch DELETE FROM account WHERE branch-name = Perryridge Delete all accounts at every branch located in Needham city. DELETE FROM account WHERE branch-name IN ( SELECT branch-name FROM branch WHERE branch-city = Needham )
78 What does the following statement do? delete from account
79 Delete: Example Delete the record of all accounts with balances below the average at the bank. delete from account where balance < (select avg (balance) from account)" Problem: as we delete tuples from account, the average balance changes Solution used in SQL: 1. First, compute avg balance and find all tuples to delete 2. Next, delete all tuples found above (without recomputing avg or retesting the tuples)
80 Inserting a Tuple into a Relation INSERT INTO R(A1,,An) VALUES (v1,,vn) A tuple is created using value vi for attribute Ai, for i=1,,n insert into account (branch-name, balance, account-number) values ( Perryridge, 1200, A-9732 ) INSERT INTO R VALUES (v1,,vn) A tuple is created using value vi for all attributes A of R Order of values must be the same as the standard order of the attributes in the relation insert into account values ( A-9732, Perryridge,1200) ---- correct order!
81 Inserting a Tuple into a Relation (cont.) insert into account (branch-name, account-number) values ( Perryridge, A-9732 ) Is equivalent to insert into account (branch-name, account-number,balance) values ( A-9732, Perryridge,NULL) If a value is omitted, it will become a NULL
82 Inserting the Results of a Query Provide as a gift for all loan customers of the Perryridge branch, a $200 savings account. Let the loan number serve as the account number for the new savings account insert into account select loan-number, branch-name, 200 from loan where branch-name = Perryridge insert into depositor select customer-name, loan-number from loan, borrower where branch-name = Perryridge and loan.account-number = borrower.account-number Set of tuples to insert
83 Order of Insertion insert into account select loan-number, branch-name, 200 from loan where branch-name = Perryridge insert into depositor select customer-name, loan-number from loan, borrower where branch-name = Perryridge and loan.account-number = borrower.account-number The select from where statement is fully evaluated before any of its results are inserted into the relation. What would happen with the following query? insert into table1 select * from table1
84 Updates Choose tuples to be updated using a query update R set attribute = expression where <condition> Pay 5% interest on accounts whose balance is greater than average update account set balance = balance * 1.05 where balance > (select avg(balance) from account)
85 Update: Example Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%. Write two update statements: update account set balance = balance * 1.06 where balance > update account set balance = balance * 1.05 where balance The order is important, why? Accounts with balance > would be updated twice! Can be done better using the case statement (next slide)
86 Case Statement for Conditional Updates Same query as before: Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%. update account set balance = case when balance <= then balance *1.05 else balance * 1.06 end
87 SQL as a Data Definition Language (DDL)
88 Data Definition Language (DDL) Allows the specification of not only a set of relations but also information about each relation, including:" The schema for each relation. The domain of values associated with each attribute. Integrity constraints The set of indices to be maintained for each relations. Security and authorization information for each relation. The physical storage structure of each relation on disk.
89 Domain Types in SQL char(n). Fixed length character string, with user-specified length n. varchar(n). Variable length character strings, with user-specified maximum length n. int. Integer (a finite subset of the integers that is machine-dependent). smallint. Small integer (a machine-dependent subset of the integer domain type). numeric(p,d). Fixed point number, with user-specified precision of p digits, with n digits to the right of decimal point. real, double precision. Floating point and double-precision floating point numbers, with machine-dependent precision. float(n). Floating point number, with user-specified precision of at least n digits. Null values are allowed in all the domain types. Declaring an attribute to be not null prohibits null values for that attribute. create domain construct in SQL-92 creates user-defined domain types create domain person-name char(20) not null
90 Date/Time Types in SQL (Cont.) date. Dates, containing a (4 digit) year, month and date E.g. date time. Time of day, in hours, minutes and seconds. E.g. time 09:00:30 time 09:00:30.75 timestamp: date plus time of day E.g. timestamp :00:30.75 Interval: period of time E.g. Interval 1 day Subtracting a date/time/timestamp value from another gives an interval value Interval values can be added to date/time/timestamp values Can extract values of individual fields from date/time/timestamp E.g. extract (year from r.starttime) Can cast string types to date/time/timestamp E.g. cast <string-valued-expression> as date
91 Create Table Construct An SQL relation is defined using the create table command: create table r (A 1 D 1, A 2 D 2,..., A n D n, (integrity-constraint 1 ),..., (integrity-constraint k )) r is the name of the relation each A i is an attribute name in the schema of relation r D i is the data type of values in the domain of attribute A i Example: create table branch (branch-name char(15) not null, branch-city char(30), assets integer)
92 Integrity Constraints in Create Table not null primary key (A 1,..., A n ) check (P), where P is a predicate P must be satisfied by all tuples Example: Declare branch-name as the primary key for branch and ensure that the values of assets are nonnegative.! "create table branch " "(branch-name"char(15),!!branch-city "char(30)," " "assets "integer, " "primary key (branch-name), " "check (assets >= 0))" primary key declaration on an attribute automatically ensures not null in SQL-92 onwards, needs to be explicitly stated in SQL-89!
93 Integrity Constraints in Create Table foreign key (A 1,..., A n ) references R Example: Create the borrower table which captures the relationship between borrower and customer, and between borrower and loan" " " create table borrower (customer_name varchar(30)," " " "loan_number number(8), " "CONSTRAINT fk1 " " FOREIGN KEY (customer_name) " " REFERENCES customer (customer_name)," "CONSTRAINT fk2 " " FOREIGN KEY (loan_number) " " REFERENCES loan )"
94 Integrity Constraints in Create Table ON DELETE CASCADE Specifies that if an attempt is made to delete a row with a key referenced by foreign keys in existing rows in other tables, all rows containing those foreign keys are also deleted. create table borrower (customer_name varchar(30), loan_number number(8), CONSTRAINT fk1 FOREIGN KEY (customer_name) REFERENCES customer (customer_name), CONSTRAINT fk2 FOREIGN KEY (loan_number) REFERENCES loan ON DELETE CASCADE)
95 Drop and Alter Table Constructs The drop table command deletes all information about the dropped relation from the database. The alter table command is used to add attributes to an existing relation. alter table r add A D where A is the name of the attribute to be added to relation r and D is the domain of A. All tuples in the relation are assigned null as the value for the new attribute. Examples: ALTER TABLE borrower ADD b_date DATE DROP TABLE borrower
96 Drop and Alter Table Constructs (cont.) The alter table command can also be used to drop attributes of a relation alter table r drop A where A is the name of an attribute of relation r E.g., ALTER TABLE borrower DROP b_date Dropping of attributes not supported by many databases The alter table command can also be used to drop or add constraints More about this later!
97 Default Values Any place we declare an attribute we may add the keyword DEFAULT followed by NULL or a constant Example: Gender CHAR(1) DEFAULT? Birthdate DATE DEFAULT DATE
98 Views Relation that is not part of the logical model create view v as <query expression> where <query expression> is any legal relational algebra query expression. The view name is represented by v Once a view is defined, its name can be used to refer to the virtual relation that the view generates View definition is not the same as creating a new relation by evaluating the query expression A view definition causes the saving of an expression; the expression is substituted into queries using the view
99 CREATE VIEW AllCustomers AS View: Examples (SELECT branch-name, cust-name FROM depositor D, account A WHERE D.account-no=A.number) UNION (SELECT branch-name, cust-name FROM borrower B, loan L WHERE B.loan-no=L.number) To find all customers of the Perryridge branch: SELECT cust-name FROM AllCustomers WHERE branch-name= Perryridge Customers with a savings account Customers with a loan account
100 Views: Renaming Attributes CREATE VIEW AllCustomers(bname,cname) AS (SELECT branch-name, cust-name FROM depositor D, account A WHERE D.account-no=A.number) UNION (SELECT branch-name, cust-name FROM borrower B, loan L WHERE B.loan-no=L.number) To find all customers of the Perryridge branch: SELECT cname FROM AllCustomers WHERE bname= Perryridge Customers with a savings account Customers with a loan account
101 Interpreting Queries that use Views To find all customers of the Perryridge branch: ----> SELECT cust-name FROM AllCustomers WHERE branch-name= Perryridge (SELECT branch-name, cust-name FROM depositor D, account A WHERE D.account-no=A.number AND branchname= Perryridge ) UNION (SELECT branch-name, cust-name The database will perform this rewriting FROM borrower B, loan L WHERE B.loan-no=L.number AND branch-name= Perryridge )
102 Create a real table Materialized Views CREATE MATERIALIZED VIEW hr.employees AS SELECT * FROM hr.employees@orc1.world;
103 Challenge Questions Are these queries equivalent? SELECT AVG(amount) FROM loan SELECT SUM(amount)/COUNT(*) FROM loan
104 Challenge Question Are these queries equivalent? SELECT SalespersonNum, AVG(CBalance) FROM Customer GROUP BY SalespersonNum HAVING AVG(CBalance) > 200; SELECT SalespersonNum, AVG(CBalance) FROM Customer WHERE CBalance > 200 GROUP BY SalespersonNum;
105 Challenge Question equivalent SELECT * FROM Customer C, Salesperson S WHERE C.SalespersonNum = S.Number; SELECT * FROM Customer C JOIN Salesperson S on SalespersonNum; SELECT * FROM Customer NATURAL JOIN Salesperson this query is not equivalent to above two queries, why? Customer (Number, Name, Address, CRating, CAmount, CBalance, SalespersonNum) Salesperson (Number, Name, Address, Office)
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