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SQL: The Complete Reference
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SELECT COUNT(NAME) FROM SALESREPS WHERE SALES > QUOTA COUNT(NAME) -----------7
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SELECT COUNT(AMOUNT) FROM ORDERS WHERE AMOUNT > 25000.00 COUNT(AMOUNT) -------------4
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Note that the COUNT() function ignores the values of the data items in the column; it simply counts how many data items there are. As a result, it doesn t really matter which column you specify as the argument of the COUNT() function. The last example could just as well have been written this way:
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SELECT COUNT(ORDER_NUM) FROM ORDERS WHERE AMOUNT > 25000.00 COUNT(ORDER_NUM) ----------------4
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In fact, it s awkward to think of the query as counting how many order amounts or counting how many order numbers ; it s much easier to think about counting how many orders. For this reason, SQL supports a special COUNT(*) column function, which counts rows rather than data values. Here is the same query, rewritten once again to use the COUNT(*) function:
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SELECT COUNT(*) FROM ORDERS
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Summary Queries
WHERE AMOUNT > 25000.00 COUNT(*) --------4
If you think of the COUNT(*) function as a rowcount function, it makes the query easier to read. In practice, the COUNT(*) function is almost always used instead of the COUNT() function to count rows.
RETRIEVING DATA
Column Functions in the Select List
Simple queries with a column function in their select list are fairly easy to understand. However, when the select list includes several column functions, or when the argument to a column function is a complex expression, the query can be harder to read and understand. The following steps show the rules for SQL query processing expanded once more to describe how column functions are handled. As before, the rules are intended to provide a precise definition of what a query means, not a description of how the DBMS actually goes about producing the query results. To generate the query results for a SELECT statement: 1. If the statement is a UNION of SELECT statements, apply Steps 2 through 5 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. For each remaining row, 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. For a column function, use the entire set of rows as its argument. 5. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. 6. 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. 7. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results.
SQL: The Complete Reference
One of the best ways to think about summary queries and column functions is to imagine the query processing broken down into two steps. First, you should imagine how the query would work without the column functions, producing many rows of detailed query results. Then you should imagine SQL applying the column functions to the detailed query results, producing a single summary row. For example, consider the following complex query: Find the average order amount, total order amount, average order amount as a percentage of the customer s credit limit, and average order amount as a percentage of the salesperson s quota.
SELECT AVG(AMOUNT), SUM(AMOUNT), (100 * AVG(AMOUNT/CREDIT_LIMIT)), (100 * AVG(AMOUNT/QUOTA)) FROM ORDERS, CUSTOMERS, SALESREPS WHERE CUST = CUST_NUM AND REP = EMPL_NUM AVG(AMOUNT) $8,256.37 SUM(AMOUNT) $247,691.00 (100*AVG(AMOUNT/CREDIT_LIMIT)) 24.45 (100*AVG(AMOUNT/QUOTA)) 2.51
------------ ------------ ------------------------------- ------------------------
Without the column functions, it would look like this:
SELECT FROM WHERE AND AMOUNT, AMOUNT, AMOUNT/CREDIT_LIMIT,AMOUNT/QUOTA ORDERS, CUSTOMERS, SALESREPS CUST = CUST_NUM AND REP = EMPL_NUM
and would produce one row of detailed query results for each order. The column functions use the columns of this detailed query results table to generate a single-row table of summary query results. A column function can appear in the select list anywhere that a column name can appear. It can, for example, be part of an expression that adds or subtracts the values of two column functions. However, the argument of a column function cannot contain another column function, because the resulting expression doesn t make sense. This rule is sometimes summarized as it s illegal to nest column functions. It s also illegal to mix column functions and ordinary column names in a select list, again because the resulting query doesn t make sense. For example, consider this query:
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