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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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Listing 8-24. Count Employees Per Department (Third Attempt) select deptno, count(e.empno) from employees e right outer join departments d using (deptno) group by deptno; DEPTNO COUNT(E.EMPNO) -------- -------------10 3 20 5 30 6 40 0 At the end of 5, you saw an example of a PL/SQL stored function to count all employees per department (Section 5.8, Listing 5-31). In that chapter, I mentioned that this counting problem is not trivial to solve in standard SQL. In Listings 8-22, 8-23, and 8-24, you see that you should indeed be careful. You need an outer join, and you should make sure to specify the correct argument for the COUNT function to get correct results.
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Caution You should be careful with the COUNT function, especially if null values might cause problems (since group functions ignore them) and you want to count row occurrences.
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Valid SELECT and GROUP BY Clause Combinations
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If your queries contain a GROUP BY clause, some syntax combinations are invalid and result in Oracle error messages, such as the following: ORA-00937: not a single-group group function. This always means that there is a mismatch between your SELECT clause and your GROUP BY clause. To demonstrate valid versus invalid syntax, Table 8-3 shows one invalid and three valid syntax examples. Table 8-3 assumes you have a table T with four columns A, B, C, and D.
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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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Table 8-3. Valid and Invalid GROUP BY Syntax Examples
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Syntax
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select a, b, max(c) from t ... group by a select a, b, max(c) from t ... group by a,b select a, count(b), min(c) from t ... group by a select count(c) from t ... group by a
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Valid
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No Yes Yes Yes
The examples in Table 8-3 illustrate the following two general rules: You do not need to select the column expression you group on (see the last example). Any column expression that is not part of the GROUP BY clause can occur only in the SELECT clause as an argument to a group function. That s why the first example is invalid.
By the way, all GROUP BY examples so far showed only column names, but you can also group on column expressions, such as in the example shown in Listing 8-25. Listing 8-25. Grouping on Column Expressions select case mod(empno,2) when 0 then 'EVEN ' else 'ODD ' end as empno sum(msal) from employees group by mod(empno,2); EMPNO SUM(MSAL) ----- --------EVEN 20225 ODD 8650 This query shows the salary sums for employees with even and odd employee numbers.
8.7 The HAVING Clause
If you aggregate rows into groups with GROUP BY, you might also want to filter your query result further by allowing only certain groups into the final query result. You can achieve this with the HAVING clause. Normally, you use the HAVING clause only following a GROUP BY clause. For example, Listing 8-26 shows information about departments with more than four employees.
RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
Listing 8-26. HAVING Clause Example select from group having deptno, count(empno) employees by deptno count(*) >= 4;
DEPTNO COUNT(EMPNO) -------- -----------20 5 30 6 However, the SQL language allows you to write queries with a HAVING clause without a preceding GROUP BY clause. In that case, Oracle assumes an implicit GROUP BY on a constant expression, just as when you use group functions in the SELECT clause without specifying a GROUP BY clause; that is, the full table is treated as a single group.
The Difference Between WHERE and HAVING
It is important to distinguish the WHERE clause from the HAVING clause. To illustrate this difference, Listing 8-27 shows a WHERE clause added to the previous query. Listing 8-27. HAVING vs. WHERE select from where group by having deptno, count(empno) employees bdate > date '1960-01-01' deptno count(*) >= 4;
DEPTNO COUNT(EMPNO) -------- -----------30 5 The WHERE condition regarding the day of birth (line 3) can be checked against individual rows of the EMPLOYEES table. On the other hand, the COUNT(*) condition (line 5) makes sense only at the group level. That s why group functions should never occur in a WHERE clause. They typically result in the following Oracle error message: ORA-00934: group function is not allowed here. You ll see this error message in Listing 8-29, caused by a classic SQL mistake, as discussed shortly.
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