c# barcode image generation library HAVING Clauses Without Group Functions in Java

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HAVING Clauses Without Group Functions
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On the other hand, valid HAVING clauses without group functions are very rare, and they should be rewritten. In Listing 8-28, the second query is much more efficient than the first one.
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Listing 8-28. HAVING Clause Without a Group Function select from group having deptno, count(*) employees by deptno deptno <= 20;
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DEPTNO COUNT(*) -------- -------10 3 20 5 select from where group deptno, count(*) employees deptno <= 20 by deptno;
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DEPTNO COUNT(*) -------- -------10 3 20 5
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A Classic SQL Mistake
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Take a look at the query in Listing 8-29. It looks very logical, doesn t it Who earns more than the average salary Listing 8-29. Error Message: Group Function Is Not Allowed Here select from where where empno employees msal > avg(msal); msal > avg(msal)
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* ERROR at line 3: ORA-00934: group function is not allowed here However, if you think in terms of tuple variables, the problem becomes obvious: the WHERE clause has only a single row as its context, turning the AVG function into something impossible to derive. You can solve this problem in many ways. Listings 8-30 and 8-31 show two suggestions. Listing 8-30. One Way to Find Who Earns More Than the Average Salary select e.empno from employees e where e.msal > (select avg(x.msal) from employees x );
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EMPNO -------7566 7698 7782 7788 7839 7902 Listing 8-31. Another Way to Find Who Earns More Than the Average Salary select from , group by , having MNR -------7566 7698 7782 7788 7839 7902 The solution in Listing 8-31 would probably not win an SQL beauty contest, but it is certainly worth further examination. This solution is based on the Cartesian product of the EMPLOYEES table with itself. Notice that it doesn t have a WHERE clause. Notice also that you group on e1.EMPNO and e1.MSAL, which allows you to refer to this column in the HAVING clause. e1.empno employees e1 employees e2 e1.empno e1.msal e1.msal > avg(e2.msal);
Grouping on Additional Columns
You sometimes need this (apparently) superfluous grouping on additional columns. For example, suppose you want to see the employee number and the employee name, followed by the total number of course registrations. The query in Listing 8-32, which could be a first attempt to solve this problem, produces an Oracle error message. Listing 8-32. Error Message: Not a GROUP BY Expression e.empno, e.ename, count(*) employees e join registrations r on (e.empno = r.attendee) group by e.empno; select e.empno, e.ename, count(*) * select from
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ERROR at line 1: ORA-00979: not a GROUP BY expression The pseudo-intermediate result in Listing 8-33 explains what went wrong here, and why you must also group on e.ENAME. Listing 8-33. Pseudo-Intermediate GROUP BY Result GROUP BY e.EMPNO e.EMPNO ======= 7369 7499 7521 7566 e.ENAME ========= {'SMITH'} {'ALLEN'} {'WARD' } ... e.INIT ... ====== {'N'} {'JAM'} ... GROUP BY e.EMPNO,e.ENAME e.EMPNO ======= 7369 7499 7521 ... e.ENAME ======== 'SMITH' 'ALLEN' ... e.INIT ... ====== {'N'} {'JAM'} ...
The two results look similar; however, there is an important difference between sets consisting of a single element, such as {'SMITH'}, and a literal value, such as 'SMITH'. In mathematics, sets with a single element are commonly referred to as singleton sets, or just singletons. Listing 8-34 shows another instructive mistake. Listing 8-34. Error Message: Not a Single-Group Group Function select deptno , sum(msal) from employees; select deptno * ERROR at line 1: ORA-00937: not a single-group group function In the absence of a GROUP BY clause, the SUM function would return a single row, while DEPTNO would produce 14 department numbers. Two columns with different row counts cannot be presented side-byside in a single result. After the correction in Listing 8-35, the error message disappears, and you get the desired results. Listing 8-35. Correction of the Error Message in Listing 8-34 select , from group by deptno sum(msal) employees deptno;
DEPTNO SUM(MSAL) -------- ------------10 8750 20 10875 30 9250
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