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Recall from 6 that you can specify the DISTINCT keyword at the beginning of the select list to eliminate duplicate rows of query results. You can also ask SQL to eliminate duplicate values from a column before applying a column function to it. To eliminate duplicate values, the keyword DISTINCT is included before the column function argument, immediately after the opening parenthesis. Here are two queries that illustrate duplicate row elimination for column functions: How many different titles are held by salespeople
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SELECT COUNT(DISTINCT TITLE) FROM SALESREPS COUNT(DISTINCT TITLE) ---------------------3
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How many sales offices have salespeople who are over quota
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SELECT COUNT(DISTINCT REP_OFFICE) FROM SALESREPS WHERE SALES > QUOTA COUNT(DISTINCT REP_OFFICE) --------------------------4
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The SQL1 standard specified that when the DISTINCT keyword is used, the argument to the column function must be a simple column name; it cannot be an expression. The standard allows the DISTINCT keyword for the SUM() and AVG() column functions. The standard does not permit use of the DISTINCT keyword with
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the MIN() and MAX() column functions because it has no impact on their results, but many SQL implementations allow it anyway. The standard also requires the DISTINCT keyword for the COUNT() column function, but many SQL implementations permit the use of the COUNT() function without it. DISTINCT cannot be specified for the COUNT(*) function, because it doesn t deal with a column of data values at all it simply counts rows. The SQL2 standard relaxed these restrictions, allowing DISTINCT to be applied for any of the column functions and permitting expressions as arguments for any of the functions as well. In addition, the DISTINCT keyword can be specified only once in a query. If it appears in the argument of one column function, it can t appear in any others. If it is specified before the select list, it can t appear in any column functions. The only exception is that DISTINCT may be specified a second time inside a subquery (contained within the query). Subqueries are described in 9.
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The summary queries described thus far are like the totals at the bottom of a report. They condense all of the detailed data in the report into a single, summary row of data. Just as subtotals are useful in printed reports, it s often convenient to summarize query results at a subtotal level. The GROUP BY clause of the SELECT statement provides this capability. The function of the GROUP BY clause is most easily understood by example. Consider these two queries: What is the average order size
SELECT AVG(AMOUNT) FROM ORDERS AVG(AMOUNT) -----------$8,256.37
What is the average order size for each salesperson
SELECT REP, AVG(AMOUNT) FROM ORDERS GROUP BY REP REP AVG(AMOUNT) ---- -----------101 $8,876.00 102 $5,694.00
8:
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103 105 106 107 108 109 110
$1,350.00 $7,865.40 $16,479.00 $11,477.33 $8,376.14 $3,552.50 $11,566.00
The first query is a simple summary query like the previous examples in this chapter. The second query produces several summary rows one row for each group, summarizing the orders taken by a single salesperson. Figure 8-3 shows how the second query works. Conceptually, SQL carries out the query as follows: 1. SQL divides the orders into groups of orders, with one group for each salesperson. Within each group, all of the orders have the same value in the REP column. 2. For each group, SQL computes the average value of the AMOUNT column for all of the rows in the group and generates a single, summary row of query results. The row contains the value of the REP column for the group and the calculated average order size.
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Figure 8-3.
A grouped query in operation
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A query that includes the GROUP BY clause is called a grouped query because it groups the data from its source tables and produces a single summary row for each row group. The columns named in the GROUP BY clause are called the grouping columns of the query, because they determine how the rows are divided into groups. Here are some additional examples of grouped queries: What is the range of assigned quotas in each office
SELECT REP_OFFICE, MIN(QUOTA), MAX(QUOTA) FROM SALESREPS GROUP BY REP_OFFICE REP_OFFICE MIN(QUOTA) MAX(QUOTA) ----------- ------------ -----------NULL NULL NULL 11 $275,000.00 $300,000.00 12 $200,000.00 $300,000.00 13 $350,000.00 $350,000.00 21 $350,000.00 $350,000.00 22 $300,000.00 $300,000.00
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