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SELECT NAME, SUM(SALES) FROM SALESREPS
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The first select item asks SQL to generate a ten-row table of detailed query results one row for each salesperson. The second select item asks SQL to generate a one-row column of summary query results containing the total of the SALES column. The two SELECT items contradict one another, producing an error. For this reason, either all column references in the select list must appear within the argument of a column function (producing a summary query), or the select list must not contain any column functions (producing a detailed query). Actually, the rule is slightly more complex when grouped queries and subqueries are considered. The necessary refinements are described later in the Group Search Conditions section.
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The SUM(), AVG(), MIN(), MAX(), and COUNT() column functions each take a column of data values as their argument and produce a single data value as a result. What happens if one or more of the data values in the column is a NULL value The ANSI/ISO SQL standard specifies that NULL values in the column are ignored by the column functions. This query shows how the COUNT() column function ignores any NULL values in a column:
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SELECT COUNT(*), COUNT(SALES), COUNT(QUOTA) FROM SALESREPS COUNT(*) COUNT(SALES) COUNT(QUOTA) --------- ------------- ------------10 10 9
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The SALESREPS table contains ten rows, so COUNT(*) returns a count of ten. The SALES column contains ten non-NULL values, so the function COUNT(SALES) also returns a count of ten. The QUOTA column is NULL for the newest salesperson. The COUNT(QUOTA) function ignores this NULL value and returns a count of nine. Because of these anomalies, the COUNT(*) function is almost always used instead of the COUNT() function, unless you specifically want to exclude NULL values in a particular column from the total. Ignoring NULL values has little impact on the MIN() and MAX() column functions. However, it can cause subtle problems for the SUM() and AVG() column functions, as illustrated by this query:
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SELECT SUM(SALES), SUM(QUOTA), (SUM(SALES) SUM(QUOTA)), SUM(SALES-QUOTA) FROM SALESREPS SUM(SALES) SUM(QUOTA) (SUM(SALES)-SUM(QUOTA)) SUM(SALES-QUOTA) -------------- -------------- ------------------------ ----------------$2,893,532.00 $2,700,000.00 $193,532.00 $117,547.00
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SQL: The Complete Reference
You would expect the two expressions:
(SUM(SALES) SUM(QUOTA)) and SUM(SALES-QUOTA)
in the select list to produce identical results, but the example shows that they do not. The salesperson with a NULL value in the QUOTA column is again the reason. The expression:
SUM(SALES)
totals the sales for all ten salespeople, while the expression:
SUM(QUOTA)
totals only the nine non-NULL quota values. The expression:
SUM(SALES) SUM(QUOTA)
computes the difference of these two amounts. However, the column function:
SUM(SALES QUOTA)
has a non-NULL argument value for only nine of the ten salespeople. In the row with a NULL quota value, the subtraction produces a NULL, which is ignored by the SUM() function. Thus, the sales for the salesperson without a quota, which are included in the previous calculation, are excluded from this calculation. Which is the correct answer Both are! The first expression calculates exactly what it says: the sum of SALES, less the sum of QUOTA. The second expression also calculates exactly what it says: the sum of (SALES QUOTA). When NULL values occur, however, the two calculations are not quite the same. The ANSI/ISO standard specifies these precise rules for handling NULL values in column functions: I If any of the data values in a column are NULL, they are ignored for the purpose of computing the column function s value. I If every data item in the column is NULL, then the SUM(), AVG(), MIN(), and MAX() column functions return a NULL value; the COUNT() function returns a value of zero.
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I If no data items are in the column (that is, the column is empty), then the SUM(), AVG(), MIN(), and MAX() column functions return a NULL value; the COUNT() function returns a value of zero. I The COUNT(*) counts rows and does not depend on the presence or absence of NULL values in a column. If there are no rows, it returns a value of zero. Although the standard is very clear in this area, commercial SQL products may produce results different from the standard, especially if all of the data values in a column are NULL or when a column function is applied to an empty table. Before assuming the behavior specified by the standard, you should test your particular DBMS.
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