Outer Joins Inner Joins Vs Outer Joins dname loc ename

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1 Outer Joins Inner Joins Vs Outer Joins When combining two tables an inner join is frequently performed to combine correct rows from the first table with correct rows from the second. In SQL this is achieved by adding a condition that matches some shared attribute in some way e.g. emp.deptno=dept.deptno, or sal>=losal and sal<=hisal) Unfortunately the way that a join is implemented in SQL a cartesian product, followed by a selection) may throw away extra rows which we actually require. Example: Get a list of the department names with location, along with the employee names who work in them. This is to include departments which do not have any staff allocated to them at present. A first incorrect) attempt may look like this: SELECT dname, loc, ename from dept, emp where emp.deptno=dept.deptno order by dname But running this query does not show the employees working in Operations as there are currently no employees in the Operations department). dname loc ename ACCOUNTING LONDON BLACK ACCOUNTING LONDON PARKER ACCOUNTING LONDON POLLARD RESEARCH YORK GIBSON RESEARCH YORK HAYES RESEARCH YORK REES RESEARCH YORK BIRD RESEARCH YORK MARCH SALES BIRMINGHAM CASSY SALES BIRMINGHAM TURNER SALES BIRMINGHAM COX SALES BIRMINGHAM AHMAD SALES BIRMINGHAM BELL SALES BIRMINGHAM BYRNE In fact we need to ask for all rows that match, plus any rows from the dept table that don t have a match i.e. the Operations department). In Relational Algebra this is called an outer 1

2 join to include rows from the Department table that don t match rows in the Employee table). In Access SQL left and right outer joins are achieved by using the following syntax: FROM table1 [ LEFT RIGHT ] JOIN table2 ON table1.field1 operator table2.field2 Where table1 and table 2 are the two tables being combined with the operator using some fields table1.field1 and table2.field2. The Left Outer Join includes extra rows from the relation on the left of the join, the Right Outer Join includes rows from the right. SELECT dname, loc, ename from dept left join emp on emp.deptno=dept.deptno Note that this can be different in other SQL implementations Oracle SQL uses +) next to the table in the where clause: SELECT dname, loc, ename from dept, emp where emp.deptno=dept.deptno +) SQL - Manipulating tables Data Definition Language DDL) The SQL language has facilities to create, manipulate and delete drop) tables. Often these command line activities are duplicated through a GUI such as the one in Access), however there are advantages to performing these operations through text. As an example consider a temporary table created, filled with records and then dropped with no user intervention. Creating a table The SQL create table syntax is of the form CREATE TABLE tablename column_name type [NULL/NOT NULL], column_name type [NULL/NOT NULL],..) 2

3 According to the relational model rules, each tablename must be unique for the database and each column_name must be unique for each relation/table. Column_name may be up to 30 characters in length starting with an initial alphabetic character. The name may consist of alphanumeric characters and the special characters _, $, # The SQL standard suggests the following types: CHAR size) DATE LONG NUMBER Character data, maximum of size characters upto 240 Dates which include time) Character data up to some restrictions may apply on the use of this field in a select statement) Maximum of 40 digits will accept scientific notation) There are other types which can be found in the SQL standard or the user guide for the particular database you are using. NULL and NOT NULL indicate whether the field will allow NULL values which is the default) or whether all cells must have a value. Note that the way that desktop databases have implemented these types varies from version to version Access 97 / 2000 offers Text, Memo, Number, Date/Time, Currency, AutoNumber, Yes/No, OLE Object and Hyperlink. Example The following DDL code will create a table called dept2 with three fields. create table dept2 deptno number not null, dname char15), loc char15) Attempting to insert a record with null for deptno into dept2 will generate an error. create table emp2 empno number, ename char15), job char15), 3

4 sal number, comm number, deptno number null This emp2 table has six fields: employee no empno), employee name ename), job title job), salary sal), commission comm) and department no deptno). NULL values will be allowed in the deptno field. Additional information about a field such as whether it s a primary or foreign key, or automatically incremented) can also be appended to a field description. It is common for a primary key value to be auto incremented so that newly created records will have a valid key field value +1 from the previous higher value). create table dept3 deptno autoincrement primary key; dname char15), loc char15) Creates a table called dept3 with a change to the deptno field so that it is an auto incremented primary key. 4

5 A correctly constructed empno key for the emp table would look like this: create table emp3 empno autoincrement primary key, ename char15), job char15), sal number, comm number, deptno number SQL has a syntax for foreign key integrity constraints that are of the form not supported in Access): fieldname type CONSTRAINT constraintname FOREIGN KEY fieldname) REFERENCES tablename Creating an emp4 table with the correct deptno attributes: create table emp4 empno autoincrement primary key, ename char15), job char15), sal number, comm number, deptno number constraint deptnocons FOREIGN KEY references dept deptno) Again, there are various levels of support for this syntax in desktop databases and some, such as Access offer an alternative graphical drag and drop approach). mysql 3.x does not support the action of foreign keys but does not explicitly reject the syntax. Deleting a table To permanently delete a table to drop a table), use the drop command: drop table tablename Note that most databases regard this as a irreversible process no undo features are typically supplied Example Deleting the dept2 table: drop table dept2 5

6 Modifying a table structure To change a table structure use the alter table command: In SQL: alter table tablename [MODIFY columnname type ADD columnname type ] ) In Access: alter table tablename [ALTER COLUMN columnname type ADD columnname type ] Examples To add a spouses_name field to the emp2 table: alter table emp2 add spouses_name char To increase the size of the ename column in SQL: alter table emp modify ename char20) In Access use ALTER COLUMN instead of the standard term modify : alter table emp alter column ename char20) Note that different databases will react in different ways if attempts are made to Delete columns where there is data present Decrease the size of a column where data is present Change NULL columns to NOT NULL Convert a column to a different type Some implementations will take unpredictable best guess solutions. 6

7 Data manipulation Language DML) Most SQL queries allow views on the original data, without manipulating the original data set. Actual changes to rows records or tuples) in a table are done through the Insert, Update or Delete statements. Inserting records into a table The insert statement adds records rows) to a table and has two forms: insert into table [columnname, columnname, )] value, value, ) values This will insert a record using a supplied column list the supplied values. If no column list is supplied the record will be inserted as is, which may generate errors if the columns don t match up. insert into table [columnname, columnname, )] select-list from tables) etc select This form allows an insert to be based on the results of a select query. Examples Inserting a new record into the dept table: insert into dept2 deptno,dname) values 50, Marketing ) If a field is left off the list but is defined as NOT NULL an error message will be generated. Note that autoincrementing key fields can be left off the insert list and the appropriate values will be calculated and pasted in. Inserting staff members from dept 10 into a table called emp2: insert into emp2 empno, ename, sal, job, hiredate) select empno,, ename, sal, job, hiredate from emp where deptno=10; Deleting records in a table The delete statement removes records rows) from a table: delete * from table [where condition] 7

8 Note that if no condition is supplied, all records may be deleted leaving an empty table structure). DELETE * FROM department WHERE deptno=10 or deptno=15 Exercises Outer Join, DDL and DML activities 1. Get all the employees and their manager, but include those staff that don t have a manager 2. Hard) Show all employees that don t manage anyone Write SQL commands to perform the following activities One Create a specialised employee table called emplondon, which has the empno, ename, job, manager and sal field names like in the employee table) Two Write an appropriate SQL query to insert a new member of staff into emplondon with the following details: Employee no: 697, Ename: Parsons, Job: Manager, Manager: 875, Sal: Three Write a query that will copy the details of other personnel who work in London) into the table use an Insert statement with an appropriate selection statement). Four Write an SQL statement to drop the emplondon table 8