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International characters (for example, accented characters in French, German, Spanish, or Italian, or the Cyrillic alphabet letters used in Greek or Russian, or the Kanji symbols used in Japanese) pose additional problems. Some brands of DBMS use special international sorting algorithms to sort these characters into their correct position for each language. Others simply sort them according to the numeric value of the code assigned to the character. To address these issues, the SQL standard includes elaborate support for national character sets, user-defined character sets, and alternate collating sequences. Unfortunately, support for these SQL features varies widely among popular DBMS products. If your application involves international text, you will want to experiment with your particular DBMS to find out how it handles these characters.
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The COUNT() column function counts the number of data values in a column. The data in the column can be of any type. The COUNT() function always returns an integer, regardless of the data type of the column. Here are some examples of queries that use the COUNT() column function: How many customers are there
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SELECT COUNT(CUST_NUM) FROM CUSTOMERS; COUNT(CUST_NUM) ---------------21
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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 that includes a column name does not count NULL values in that column, but COUNT(*) counts all rows regardless of column values. Aside from NULL values, however, COUNT() 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
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8:
Summary Queries
column you specify as the argument of the COUNT() function. The last example could just as well have been written this way:
SELECT COUNT(ORDER_NUM) FROM ORDERS WHERE AMOUNT > 25000.00; COUNT(ORDER_NUM) ----------------4
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:
SELECT COUNT(*) FROM ORDERS WHERE AMOUNT > 25000.00; COUNT(*) --------4
PART II
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.
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.
Part II:
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