CS2 Current Technologies Note 3 CS2Bh

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1 CS2 Current Technologies Note 3 SQL: Joins and Subqueries Introduction All of the queries which we saw in the previous lecture could be answered by consulting either the DEPT or the EMP table. There are, however, other queries where this is not sufficient. Let s say we would like to know all employees working in Dallas. One way to deal with this is to use two queries. First, we find out the department number of the department located in Dallas; a query on table DEPT only. Then we search for employees with the previously found department number; a query on table EMP only. Although this is a correct solution, it is somewhat cumbersome, since we need to issue two queries to answer our question. SQL provides a mechanism, the join query, which allows us to consult more than one table within a single SELECT statement. Joins: Data from Multiple Tables A join-query is a query with more than one table listed in the FROM clause. If no particular columns and rows are selected, SQL produces an output which consists of all possible concatenations of all rows of tables given in the query. For example, table EMP consists of 14 rows, with 8 columns per row, and table DEPT consists of 4 rows, with 3 columns per row. An unrestricted join of these two tables would lead to 56 rows, with each 11 columns. Such unrestricted join-queries are usually not what we want. Often, tables have a particular relation with each other. In our example, that relation is given through the department number. Through the department number we can determine information about an employee s department. To maintain this particular relation we have to specify a join-condition. The join condition for our example is specified in the WHERE clause as EMP.DEPTNO = DEPT.DEPTNO. Notice that we used a dot notation to distinguish between columns with equal names. Example 1 Which employees work in Dallas? SELECT ENAME FROM EMP, DEPT WHERE EMP.DEPTNO = DEPT.DEPTNO AND LOC = DALLAS ; 1

2 ENAME SMITH ADAMS FORD SCOTT JONES Rows in one table may be joined to rows in another by common values in corresponding columns. In the above example, the tables EMP and DEPT both have a column, DEPTNO, that contains department numbers. These columns allow you to join rows in the two tables. Suppose that you want to know where the employee named ALLEN is located. Note that the table EMP does not contain a location (LOC) column, but the table DEPT does contain a LOC column. By looking at EMP, you can see that ALLEN works in Department 30. By looking at DEPT, you can see that Department 30 is located in CHICAGO. Thus to join a row in EMP to a row in DEPT, you use the value in the DEPTNO column in each table in this case 30. But in general, the user may not know the relevant value or values that are to be used in the join. All that the user needs to know is that the general method for associating a location with an employee is to use the DEPTNO column. The SQL query is given in Example 2. Example 2 Where does ALLEN work? SELECT ENAME, LOC FROM EMP, DEPT WHERE EMP.DEPTNO = DEPT.DEPTNO AND ENAME = ALLEN ; ENAME ALLEN LOC CHICAGO Specifying Join Columns in a WHERE Clause To make a query in which rows of two tables are joined, you must specify the join columns that contain corresponding information in the two tables. Specify the tables to be joined in the FROM clause and specify the join columns in the WHERE clause. The names of the tables are separated by commas. The logical expression specifies how the tables are to be joined. It refers to the join columns in the two tables and its value is true for those combinations of rows that are to be joined. The WHERE clause may be used to specify selection conditions as well as join columns. In this case, use AND to combine the logical expressions that select rows and the logical expressions that define join conditions. 2

3 In Example 2, the first logical expression ( EMP.DEPTNO = DEPT.DEPTNO ) is the join condition and the second logical expression makes the query retrieve only those rows for which the ENAME field has the value ALLEN. (There is only one such row in this example, but there could be several employees named ALLEN in general. In this case, more than one row is retrieved.) Referring to Tables In Example 2, the DEPTNO column name is prefixed by the table name EMP or DEPT and a period. This is because both EMP and DEPT have a column named DEPTNO. It is not permitted to give two columns the same name when they are in the same table. When two columns have the same name, you must use table name prefixes to show which column you mean. If a column name is unique in the tables included in the query, you need not prefix it. Thus, for example, there is no need to prefix ENAME. Incidentally, in Example 2 the column names used in the join condition happened to be the same. In general, this need not be the case. Abbreviating a Table Name Although table name prefixes prevent ambiguity in a query, they can be tedious to enter. You can define temporary labels in the FROM clause and use them elsewhere in the query. Such temporary labels are called aliases. Note that you can use the alias in the SELECT line, even though it is only introduced in the FROM line. This is illustrated in Example 3 Example 3 Use of Aliases: SELECT D.* FROM DEPT D; Equi-Joins and Non-Equi-Joins The comparison operator used in the join condition in Example 2 was the equals sign, =. Such join queries are called equi-joins. A non-equi-join is a query that specifies some relationship other than equality between the columns. Suppose that you want to find all the employees who earn more than JONES. Example 4 illustrates how to use a non-equi-join to answer this query. Example 4 A non-equi-join query. SELECT X.ENAME, X.SAL FROM EMP X, EMP Y WHERE X.SAL > Y.SAL AND Y.ENAME = JONES ; 3

4 In Example 4, the EMP table is joined with itself using the comparison operator > (greater than). We set up the aliases X and Y for the table EMP. Each row in X is appended to JONES row in Y if the SALary is greater than JONES SALary. Note that, in general, a row in one table may be joined to many rows in another table. Thus the output can be huge compared to the size of the original tables unless some other selection condition is applied in order to reduce the number of rows. Outer Joins If a row in one of the tables does not satisfy the join condition, that row ordinarily will not appear in the query s result. In the running example, department 40 ( OPERATIONS ) has no employees. If in a query you want to display those rows from DEPT that have no matching employees, you may use the outer join operator, a plus sign enclosed in parentheses, (+). This is illustrated in Example 5. In this example, we want to list all departments which do not have any employees. Example 5 The use of the outer-join operator (+). SELECT DISTINCT DEPT.DEPTNO, DNAME, LOC FROM DEPT, EMP WHERE DEPT.DEPTNO = EMP.DEPTNO (+) AND EMPNO IS NULL; DEPTNO DNAME LOC 40 OPERATIONS BOSTON Joining a Table with Itself The use of aliases allows us to specify that we want to join a table with itself as though it were two separate copies of the same table. This is useful when you want to join one row of the table with another row of the same table. Example 6 illustrates this. Example 6 Which employees have a salary which exceeds the salary of their manager? SELECT W.ENAME, W.SAL, M.ENAME "MNAME", M.SAL "MSAL" FROM EMP W, EMP M WHERE W.MGR = M.EMPNO AND W.SAL > M.SAL; 4

5 ENAME SAL MNAME MSAL SCOTT 3000 JONES 2975 FORD 3000 JONES 2975 Example 6 treats EMP as if it were two separate tables named W and M. First, all W s rows are joined to their corresponding M rows through the join condition W.MGR = M.EMPNO. (SCOTT s MGR is 7566 and EMPNO 7566 corresponds to JONES.) In effect, this associates each employee with his/her manager. The second logical expression selects worker/manager pairs where the worker earns more than the manager (W.SAL > M.SAL). Multiple Join Conditions and Tables In the above examples, we joined two tables by examining the values in one pair or columns, one column from each table. We could specify more complex join conditions. For example, we could join two tables with two AND ed conditions requiring equality in two pairs of columns. Similarly, a query may need to join more than two tables in order to answer a query. Subqueries SQL allows us to specify queries (subqueries) in the WHERE clause of other queries. This allows us to construct very powerful complex queries which we could otherwise not express easily. Consider the query in Example 7. We answer this question using two queries: the first one determines the average salary and the second selects all jobs with a salary above this average. Example 7 Which positions are paid a higher than average salary? SELECT DISTINCT JOB FROM EMP WHERE SAL > (SELECT AVG(SAL) FROM EMP); JOB ANALYST MANAGER PRESIDENT SQL processes a subquery before it processes the main query. This is necessary since the main query uses the results of the subquery for its own processing. 5

6 Subqueries can themselves have other subqueries, and they can be as complex using all clauses we have seen above as the main query. In Example 7, the main query expects a single value to be passed back by the subquery. The SAL value is to be compared to a single number, namely the average (arithmetic mean) salary. If the subquery returns no numbers or more than one number, SQL will generate an error message. You can reference columns in the outer query from the inner query even if the same table is used in both. However, if the same table is used, the table name in the outer query may have to be prefixed by an alias to enable SQL to distinguish between the two copies of the table. Subqueries that Return a Set of Values If the subquery returns more than one value, you must specify how the returned values should be used in the WHERE clause of the outer query. You should insert one of the keywords ANY and ALL between the comparison operator and the subquery. The choice of the keyword ANY is unfortunate, as in common parlance, people often say any when they mean all. See Example 8. Example 8 Which employees earn more than ANY (i.e., at least one) employee in Department 30? SELECT SAL, ENAME FROM EMP WHERE SAL > ANY (SELECT SAL FROM EMP WHERE DEPTNO = 30) ORDER BY SAL DESC; To increase readability, SQL lets you use IN for = ANY and NOT IN for!= ALL. Subqueries that Return more than one Column When a subquery returns more than one column, you must put parentheses around the list of columns on the other side of the comparison operator. Multiple Subqueries As we already know, subqueries can be nested within subqueries. Furthermore, the WHERE clause of the outer query can contain several conditions connected by the operators AND and OR; each such condition may contain subqueries. A query may be composed of two or more queries with the operators UNION, INTERSECT and EXCEPT. 6

7 UNION returns all distinct rows returned by either subquery. INTERSECT returns all rows returned by both subqueries. EXCEPT returns all rows returned by the preceding query but not by the following query. Example 9 Display the names of all employees who have a JOB category such that there are employees in that category in both Department 20 and Department 30. SELECT JOB, ENAME, DEPTNO FROM EMP WHERE JOB IN (SELECT JOB FROM EMP WHERE DEPTNO = 20 INTERSECTION SELECT JOB FROM EMP WHERE DEPTNO = 30); Correlated Subqueries So far, the examples of subqueries that we have seen have in common that each subquery is executed once and the resulting value (or set of values) was used by the WHERE clause in the containing query. It is also possible to compose a subquery that is executed repeatedly, once for each candidate row considered for selection by the outer query. Consider the following example. Example 10 Find all employees who have a salary that is more than the average salary of the employees in their own department, and list them in department order. SELECT DEPTNO, ENAME, SAL FROM EMP X WHERE SAL > (SELECT AVG(SAL) FROM EMP WHERE X.DEPTNO = DEPTNO) ORDER BY DEPTNO; Note the table alias, X, which appears in the main query. It is used in the WHERE clause of the subquery to refer to a candidate row s value of DEPTNO. The unqualified reference to DEPTNO on the right-hand side of the = sign refers to the DEPTNO field in each row of the table. Thus, when the subquery is executed for each of the main query s candidate rows, SQL compares the value of DEPTNO in the candidate row to the value of DEPTNO in each row of the table. The subquery selects each row for which the candidate row s department number equals the selected row s department number. It returns the value of AVG(SAL) computed over the selected rows. Bear in mind the following two points concerning correlated subqueries. 7

8 A correlated subquery refers to a column selected by the main query (outside the subquery). If the subquery selects from the same table as the main query, the main query must define an alias for the table name, and the subquery must use the alias to refer to the column s values in the main query s candidate rows. Subqueries that Test for Existence The keyword EXISTS is used in the WHERE clause before a subquery. If the subquery returns at least one row, then the EXISTS (subquery) condition evaluates to true. Otherwise, it evaluates to false. Example 11 Display information about employees who have at least one other employee reporting to them. (Note that this is not the same as selecting all employees whose job is MANAGER.) SELECT JOB, ENAME, EMPNO, DEPTNO FROM EMP X WHERE EXISTS (SELECT * FROM EMP WHERE X.EMPNO = MGR) ORDER BY EMPNO; Further Reading Elmasri and Navathe, chapter 8, section 8.2 and 8.3. Ramakrishnan and Gehrke, Database Management Systems, chapter 5, section 4. 8