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Working with Functions and Value Expressions
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ARTIST_NAME: VARCHAR(60) Jennifer Warnes Joni Mitchell Joni Mitchell William Ackerman Bing Crosby Patsy Cline John Barry Leonard Cohen Bonnie Raitt B.B. King
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CD_NAME: VARCHAR(60) Famous Blue Raincoat Blue Court and Spark Past Light That Christmas Feeling Patsy Cline: 12 Greatest Hits Out of Africa Leonard Cohen The Best of Fundamental Blues on the Bayou
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NUMBER_SOLD: INT 23 45 34 12 34 54 23 20 29 18
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Figure 10-1
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Using set functions on the ARTIST_CDS table
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Using the COUNT Function
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The first set function that we ll look at is the COUNT function. As the name implies, the COUNT function counts the number of rows in a table or the number of values in a column, as specified in a SELECT statement. When you use the COUNT function, you must specify a column name to count the number of non-null values in a column, or an asterisk to count all the rows in a table regardless of null values. For example, if you want to know the total number of rows in the ARTIST_CDS table, you can use the following SELECT statement:
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SELECT COUNT(*) AS TOTAL_ROWS FROM ARTIST_CDS;
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In this statement, the COUNT function is used with an asterisk in parentheses to count every row in the ARTIST_CDS table and return the total count. The returned value is listed in the TOTAL_ROWS column, a name given to the column returned in the query results, as shown in the following results:
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TOTAL_ROWS --------10
As you can see, the query results include only one value (one row with one column), as can be expected of a set function used without any row grouping. The value of 10 indicates that the ARTIST_CDS table contains 10 rows.
SQL: A Beginner s Guide
As with any other sort of SELECT statement, you can qualify your query results by adding the necessary clauses to the statement. For example, suppose you want to find out how many rows include a NUMBER_SOLD value greater than 20. You can modify your SELECT statement to include a WHERE clause:
SELECT COUNT(*) AS TOTAL_ROWS FROM ARTIST_CDS WHERE NUMBER_SOLD > 20;
The value returned will now be 7, rather than 10, because only seven rows meet the search condition specified in the WHERE clause. You might find that, instead of querying the number of rows in a table, you want to know the number of values in a given column (excluding null values). In this case, you would specify the column name rather than the asterisk. For example, suppose you modify the SELECT statement shown in the last example to count values in the ARTIST_NAME column:
SELECT COUNT(ARTIST_NAME) AS TOTAL_ARTISTS FROM ARTIST_CDS WHERE NUMBER_SOLD > 20;
When you execute this query, the value returned is again 7. This means that seven ARTIST_NAME values have a NUMBER_SOLD value greater than 20. However, this statement doesn t account for ARTIST_NAME values that might be duplicated. If you want to arrive at a count that takes into consideration duplicate values, you can add the DISTINCT keyword to the COUNT function:
SELECT COUNT(DISTINCT ARTIST_NAME) AS TOTAL_ARTISTS FROM ARTIST_CDS WHERE NUMBER_SOLD > 20;
This time, a value of 6 is returned rather than 7. This is because the ARTIST_NAME column includes two instances of the Joni Mitchell value. The column contains only six unique values that meet the condition set forth in the search criteria.
NOTE
Keep in mind that the SELECT statement is processed in a specific order: first the FROM clause, then the WHERE clause, and then the SELECT clause. As a result, the COUNT function applies only to the rows that meet the search condition defined in the WHERE clause. Rows that are not included in the results of the WHERE clause have no bearing on the COUNT function. For more information about the SELECT statement, see 7.
As already mentioned, if the column specified in the COUNT function contains null values, those values are not included in the count. For example, if we were to add a row to the ARTIST_ CDS table with an ARTIST_NAME value of null and a NUMBER_SOLD value greater than 20, the SELECT statement shown in the previous example would still return a value of 6 because the null value would not be counted. However, if you use an asterisk rather than a column name in the COUNT function, all rows are counted, even if some contain null values.
10:
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