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What is the average order size for each salesperson
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SELECT REP, AVG(AMOUNT) FROM ORDERS GROUP BY REP; REP AVG(AMOUNT) ---- -----------101 $8,876.00 102 $5,694.00 103 $1,350.00 105 $7,865.40 106 $16,479.00 107 $11,477.33 108 $8,376.14 109 $3,552.50 110 $11,566.00
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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.
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ORDERS Table
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GROUPED Table ORDER_NUM
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AMOUNT
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QUERY Results REP AVG (AMOUNT)
106 103 108 $16,479.00 $1,350.00
GROUP BY
108 108 108 108 108 108 108
$1,420.00 $45,000.00 $652.00 $7,100.00 $2,925.00 $760.00 $776.00
$8,376.14
FIGURE 8-3
A grouped query in operation
8:
Summary Queries
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. 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
PART II
How many salespeople are assigned to each office
SELECT REP_OFFICE, COUNT(*) FROM SALESREPS GROUP BY REP_OFFICE; REP_OFFICE COUNT(*) ----------- --------NULL 1 11 2 12 3 13 1 21 2 22 1
How many different customers are served by each salesperson
SELECT COUNT(DISTINCT CUST_NUM), 'customers for salesrep', CUST_REP FROM CUSTOMERS GROUP BY CUST_REP; COUNT(DISTINCT CUST_NUM) CUSTOMERS FOR SALESREP CUST_REP ------------------------- ----------------------- --------3 customers for salesrep 101 4 customers for salesrep 102 3 customers for salesrep 103 1 customers for salesrep 104
Part II:
Retrieving Data
2 2 . . .
customers for salesrep customers for salesrep
105 106
There is an intimate link between the SQL column functions and the GROUP BY clause. Remember that the column functions take a column of data values and produce a single result. When the GROUP BY clause is present, it tells SQL to divide the detailed query results into groups and to apply the column function separately to each group, producing a single result for each group. The following steps show the rules for SQL query processing, expanded once again for grouped queries. To generate the query results for a SELECT statement: 1. If the statement is a UNION of SELECT statements, apply Steps 2 through 7 to each of the statements to generate their individual query results. 2. Form the product of the tables named in the FROM clause. If the FROM clause names a single table, the product is that table. 3. If there is a WHERE clause, apply its search condition to each row of the product table, retaining those rows for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). 4. If there is a GROUP BY clause, arrange the remaining rows of the product table into row groups, so that the rows in each group have identical values in all of the grouping columns. 5. If there is a HAVING clause, apply its search condition to each row group, retaining those groups for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). 6. For each remaining row (or row group), calculate the value of each item in the select list to produce a single row of query results. For a simple column reference, use the value of the column in the current row (or row group). For a column function, use the current row group as its argument if GROUP BY is specified; otherwise, use the entire set of rows. 7. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. 8. If the statement is a UNION of SELECT statements, merge the query results for the individual statements into a single table of query results. Eliminate duplicate rows unless UNION ALL is specified. 9. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results.
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