Chapter 12: When One Query Depends Upon Another

1. Chapter 12: When One Query Depends Upon Another • Correlated Subqueries. Subqueries in the where clause. • Anti-Joins. Using NOT IN or NOT EXISTS with a subquery. • Outer Joins. Special type of join between tables where matching data not found. • Union, Intersect, Minus. Ways of combining result sets from separate queries together. CSCI N311: Oracle Database Programming

2. Chapter 12: When One Query Depends Upon Another • Regular Joins, Review.Example from p. 242, a 3 table equi-join • SELECT W.Name, W.Lodging • FROM Worker W, Workerskill WS, Lodging L • WHERE W.Name = WS.Name • and W.Lodging = L.Lodging • and Skill = ‘COMBINE DRIVER’ • and Address LIKE ‘%EDMESTON%’;

3. Chapter 12: When One Query Depends Upon Another • Correlated Subqueries. • A correlated subquery is a query in which the subquery refers to values in the parent query. • A correlated subquery can return the same results as a join, but can be used where a join cannot • update, insert and delete statements. • Group by queries • Example from book, p.245:

4. Chapter 12: When One Query Depends Upon Another • Another Corrolated Example: • The following statement returns data about employees whose salaries exceed the averages for their departments. The following statement assigns an alias to EMP, the table containing the salary information, and then uses the alias in a correlated subquery: • SELECT deptno, ename, sal • FROM emp x • WHERE sal > (SELECT AVG(sal) • FROM emp • WHERE x.deptno = deptno) • ORDER BY deptno • For each row of the EMP table, the parent query uses the correlated subquery to compute the average salary for members of the same department. The correlated subquery performs these steps for each row of the EMP table: • 1. The DEPTNO of the row is determined. • 2. The DEPTNO is then used to evaluate the parent query. • 3. If that row’s salary is greater than the average salary for that row’s department, then the row is returned. • The subquery is evaluated once for each row of the EMP table.

5. Chapter 12: When One Query Depends Upon Another • Exists: • The EXISTS keyword is similar IN. • EXISTS tests for the existence of any row. Unlike IN however, EXISTS does not match columns and it usually only makes sense to use with a correlated subquery. • Example from p. 250 • select Name, Skill • from WORKERSKILL • where EXISTS ( • select Name from WORKERSKILL group by Name • having COUNT(Skill) > 1);

6. Chapter 12: When One Query Depends Upon Another • Not Exists: • very similar to NOT IN • Much faster! Not exists can use indexes. • Example from p. 255 • select S.Skill • from SKILL S • where NOT EXISTS ( • select ‘whatever’ from WORKERSKILL WS where WS.Skill = S.Skill); • Equivalent NOT IN query • select S.Skill • from SKILL S • where S.Skill NOT IN ( • select WS.SKILL from WORKERSKILL WS);

7. Chapter 12: When One Query Depends Upon Another • Outer Join: • The outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and those rows from one table for which no rows from the other satisfy the join condition. Such rows are not returned by a simple join. • To write a query that performs an outer join of tables A and B and returns all rows from A, apply the outer join operator (+) to all columns of B in the join condition. • For all rows in A that have no matching rows in B, Oracle returns NULL for any select list expressions containing columns of B. • This is the basic syntax of an outer join of two tables: • SELECT table1.column • FROM table1, table2 • WHERE table1.column = table2.column(+)

8. Chapter 12: When One Query Depends Upon Another • Outer Join Example: • You want a list of all workers and their skills. If a worker has no skills, you want him listed without corresponding skills. • Equi-join query: • NAME SKILL • ------------------------- ------------------------- • ADAH TALBOT WORK • DICK JONES SMITHY • ELBERT TALBOT DISCUS • HELEN BRANDT COMBINE DRIVER • JOHN PEARSON COMBINE DRIVER • JOHN PEARSON WOODCUTTER • JOHN PEARSON SMITHY • VICTORIA LYNN SMITHY • WILFRED LOWELL WORK • WILFRED LOWELL DISCUS • Outer Join Query: • NAME SKILL • ------------------------- ------------------------- • ADAH TALBOT WORK • ANDREW DYE • BART SARJEANT • DICK JONES SMITHY • DONALD ROLLO • ELBERT TALBOT DISCUS • GEORGE OSCAR • GERHARDT KENTGEN • HELEN BRANDT COMBINE DRIVER • JED HOPKINS • JOHN PEARSON COMBINE DRIVER • JOHN PEARSON WOODCUTTER • JOHN PEARSON SMITHY • KAY AND PALMER WALLBOM • PAT LAVAY • PETER LAWSON • RICHARD KOCH AND BROTHERS • ROLAND BRANDT • VICTORIA LYNN SMITHY • WILFRED LOWELL WORK • WILFRED LOWELL DISCUS • WILLIAM SWING • SELECT W.Name, S.Skill FROM Worker W, • WorkerSkill S • WHERE W.Name = S.Name; • SELECT W.Name, S.Skill FROM Worker W, • WorkerSkill S • WHERE W.Name = S.Name(+);

9. Chapter 12: When One Query Depends Upon Another • Outer Join: • Outer join queries are subject to the following rules and restrictions: • The (+) operator can only appear in the WHERE clause, not in the select list, and can only be applied to a column of a table or view. • If A and B are joined by multiple join conditions, the (+) operator must be used in all of these conditions. • The (+) operator can only be applied to a column, rather than to an arbitrary expression, although an arbitrary expression can contain a column marked with the (+) operator. • A condition containing the (+) operator cannot be combined with another condition using the OR logical operator. • A condition cannot use the IN comparison operator to compare a column marked with the (+) operator to another expression. • A condition cannot compare a column marked with the (+) operator to a subquery. • If the WHERE clause contains a condition that compares a column from table B to a constant, the (+) operator must be applied to the column so that the rows from table A for which Oracle has generated NULLs for this column are returned. • In a query that performs outer joins of more than two pairs of tables, a single table can only be the NULL–generated table for one other table. For this reason, you cannot apply the (+) operator to columns of B in the join condition for A and B and the join condition for B and C.

10. Chapter 12: When One Query Depends Upon Another • Replacing NOT IN with an Outer Join • Example similar to p. 253 • Using NOT IN: • select A.Name, A.Lodging from Worker A where A.Name NOT IN (select B.Name from WORKERSKILL B where B.skill = ‘SMITHY’); • Using Outer Join: • select A.Name, A.Lodging from Worker A, WORKERSKILL B • where A.Name = B.Name(+) • and B.Name IS NULL • and B.Skill(+) = ‘SMITHY’;

11. Chapter 12: When One Query Depends Upon Another • Set Operators: • UNION returns distinct rows for the combination of two select statements. • UNION ALL returns all rows for the combination of two select statements regardless of duplication. • INTERSECT returns distinct rows for the combination of two select statements where data matches. • MINUS return the rows from one select statement excluding the rows of a second select statement.

12. Chapter 12: When One Query Depends Upon Another • Set Operations Example: • An obvious example:select Name from Longtime minusselect Name from Prospect • A subtle example:select Name, Lodging from Longtime minusselect Name, Address from Prospect • Columns must be compatible for set operations, but not necessarily the same.

13. Chapter 13: Complex Possibilities • Creating Complex Views. View of a group, view of a total, combined view.pp. 268-273 • FROM Clause Subqueries.Essentially temporary views. pp 273-4 • Temporary Tables, ROLLUP, GROUPING, CUBE.New Oracle8i features. pp. 275-9 • Hierarchical Queries.START WITH and CONNECT BY keywords. pp. 279-288

14. Chapter 13: Complex Possibilities • FROM Clause Subqueries. • New feature as of Oracle 7.3. • Works just like a view. • Example similar to p. 274 • SELECT L1.Person, L1.Amount, 100*L1.Amount/T.Total • FROM Ledger L1,(Select SUM(Amount) Total from Ledger • where Action IN (‘BOUGHT’,‘PAID’)) T • WHERE Action IN (‘BOUGHT’,‘PAID’);

15. Chapter 17: DECODE • Decode is Oracle non-standard SQL. Extremely powerful, yet underutilized. • Decode works similar to if/then/else • DECODE (value, if1, then1, if2, then2,... , default) • Common uses illustrated in the text: • Aggregating groups of data into a single column. Example, p. 353 • Flip a table on its side, ie rows become columns. Example, p. 358 • Dividing data into sections based on row number. This is done in conjunction with the MOD function. Example, p. 359 • Complex computations that require if/then logic. Example, p. 362