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Multiple Grouping Columns
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SQL can group query results based on the contents of two or more columns. For example, suppose you want to group the orders by salesperson and by customer. This query groups the data based on both criteria: Calculate the total orders for each customer of each salesperson. SELECT REP, CUST, SUM(AMOUNT) FROM ORDERS GROUP BY REP, CUST REP ---101 101 101 102 102 102 103 105 105 . . . CUST SUM(AMOUNT) ---- ----------2102 $3,978.00 2108 $150.00 2113 $22,500.00 2106 $4,026.00 2114 $15,000.00 2120 $3,750.00 2111 $2,700.00 2103 $35,582.00 2111 $3,745.00
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Even with multiple grouping columns, SQL provides only a single level of grouping. The query produces a separate summary row for each salesperson/customer pair. It's impossible to create groups and subgroups with two levels of subtotals in SQL. The best you can do is sort the data so that the rows of query results appear in the appropriate order. In many SQL implementations, the GROUP BY clause will automatically have the side effect of sorting the data, but you can override this sort with an ORDER BY clause, as shown here: Calculate the total orders for each customer of each salesperson, sorted by customer, and within each customer by salesperson. SELECT CUST, REP, SUM(AMOUNT) FROM ORDERS GROUP BY CUST, REP ORDER BY CUST, REP
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CUST ----2101 2102 2103 2106 2107 2108 2108 2109 2111 2111 . . .
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REP SUM(AMOUNT) --- ----------106 $1,458.00 101 $3,978.00 105 $35,582.00 102 $4,026.00 110 $23,132.00 101 $150.00 109 $7,105.00 107 $31,350.00 103 $2,700.00 105 $3,745.00
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Note that it's also impossible to get both detailed and summary query results from a single query. To get detailed query results with subtotals or to get multilevel subtotals, you must write an application program using programmatic SQL and compute the subtotals within the program logic. SQL Server addresses this limitation of standard SQL by adding an optional COMPUTE clause to the end of the SELECT statement. The COMPUTE clause calculates subtotals and sub-subtotals as shown in this example: Calculate the total orders for each customer of each salesperson, sorted by salesperson, and within each salesperson by customer. SELECT REP, CUST, AMOUNT FROM ORDERS ORDER BY REP, CUST COMPUTE SUM(AMOUNT) BY REP, CUST COMPUTE SUM(AMOUNT), AVG(AMOUNT) BY REP REP CUST ---- ---101 2102 AMOUNT ------------$3,978.00 sum -------------$3,978.00 AMOUNT ------------$150.00 sum -------------$150.00 AMOUNT ------------$22,500.00 sum -------------$22,500.00
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REP CUST ---- ---101 2108
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REP CUST ---- ---101 2113
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sum -------------$26,628.00 avg -------------$8,876.00 REP CUST ---- ---102 2106 102 2106 AMOUNT ------------$2,130.00 $1,896.00 sum -------------$4,026.00 AMOUNT ------------$15,000.00 sum -------------$15,000.00 AMOUNT ------------$3,750.00 sum -------------$3,750.00 sum -------------$22,776.00 avg -------------$5,694.00
REP CUST ---- ---102 2114
REP CUST ---- ---102 2120
. . . The query produces one row of detailed query results for each row of the ORDERS table, sorted by CUST within REP. In addition, it computes the sum of the orders for each customer/salesperson pair (a low-level subtotal) and computes the sum of the orders and average order size for each salesperson (a high-level subtotal). The query results thus contain a mixture of detail rows and summary rows, which include both subtotals and sub-subtotals. The COMPUTE clause is very nonstandard, and in fact it is unique to the Transact-SQL dialect used by SQL Server. Furthermore, it violates the basic principles of relational queries because the results of the SELECT statement are not a table, but a strange combination of different types of rows. Nonetheless, as the example shows, it can be very useful.
Restrictions on Grouped Queries
Grouped queries are subject to some rather strict limitations. The grouping columns must
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be actual columns of the tables named in the FROM clause of the query. You cannot group the rows based on the value of a calculated expression. There are also restrictions on the items that can appear in the select list of a grouped query. All of the items in the select list must have a single value for each group of rows. Basically, this means that a select item in a grouped query can be: a constant, a column function, which produces a single value summarizing the rows in the group, a grouping column, which by definition has the same value in every row of the group, or an expression involving combinations of the above. In practice, a grouped query will always include both a grouping column and a column function in its select list. If no column function appears, the query can be expressed more simply using SELECT DISTINCT, without GROUP BY. Conversely, if you don't include a grouping column in the query results, you won't be able to tell which row of query results came from which group! Another limitation of grouped queries is that SQL ignores information about primary keys and foreign keys when analyzing the validity of a grouped query. Consider this query: Calculate the total orders for each salesperson. SELECT EMPL_NUM, NAME, SUM(AMOUNT) FROM ORDERS, SALESREPS WHERE REP = EMPL_NUM GROUP BY EMPL_NUM Error: "NAME" not a GROUP BY expression Given the nature of the data, the query makes perfectly good sense because grouping on the salesperson's employee number is in effect the same as grouping on the salesperson's name. More precisely, EMPL_NUM, the grouping column, is the primary key of the SALESREPS table, so the NAME column must be single-valued for each group. Nonetheless, SQL reports an error because the NAME column is not explicitly specified as a grouping column. To correct the problem, you simply include the NAME column as a second (redundant) grouping column: Calculate the total orders for each salesperson. SELECT EMPL_NUM, NAME, SUM(AMOUNT) FROM ORDERS, SALESREPS WHERE REP = EMPL_NUM GROUP BY EMPL_NUM, NAME EMPL_NUM NAME ----------------------101 Dan Roberts 102 Sue Smith 103 Paul Cruz 105 Bill Adams SUM(AMOUNT) ----------$26,628.00 $22,776.00 $2,700.00 $39,327.00
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