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Restrictions on Group Search Conditions
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The HAVING clause is used to include or exclude row groups from the query results, so the search condition it specifies must be one that applies to the group as a whole rather
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Summary Queries
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than to individual rows. This means that an item appearing within the search condition in a HAVING clause can be: I A constant I A column function, which produces a single value summarizing the rows in the group I A grouping column, which by definition has the same value in every row of the group I An expression involving combinations of these In practice, the search condition in the HAVING clause will always include at least one column function. If it did not, the search condition could be moved to the WHERE clause and applied to individual rows. The easiest way to figure out whether a search condition belongs in the WHERE clause or in the HAVING clause is to remember how the two clauses are applied: I The WHERE clause is applied to individual rows, so the expressions it contains must be computable for individual rows. I The HAVING clause is applied to row groups, so the expressions it contains must be computable for a group of rows.
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NULL Values and Group Search Conditions
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Like the search condition in the WHERE clause, the HAVING clause search condition can produce one of three results: I If the search condition is TRUE, the row group is retained, and it contributes a summary row to the query results. I If the search condition is FALSE, the row group is discarded, and it does not contribute a summary row to the query results. I If the search condition is NULL, the row group is discarded, and it does not contribute a summary row to the query results. The anomalies that can occur with NULL values in the search condition are the same as those for the WHERE clause and have been described in 6.
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HAVING Without GROUP BY
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The HAVING clause is almost always used in conjunction with the GROUP BY clause, but the syntax of the SELECT statement does not require it. If a HAVING clause appears without a GROUP BY clause, SQL considers the entire set of detailed query results to be a single group. In other words, the column functions in the HAVING clause are applied to one, and only one, group to determine whether the group is included or excluded from the query results, and that group consists of all the rows. The use of a HAVING clause without a corresponding GROUP BY clause is seldom seen in practice.
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SQL: The Complete Reference
Summary
This chapter described summary queries, which summarize data from the database: I Summary queries use SQL column functions to collapse a column of data values into a single value that summarizes the column. I Column functions can compute the average, sum, minimum, and maximum values of a column, count the number of data values in a column, or count the number of rows of query results. I A summary query without a GROUP BY clause generates a single row of query results, summarizing all the rows of a table or a joined set of tables. I A summary query with a GROUP BY clause generates multiple rows of query results, each summarizing the rows in a particular group. I The HAVING clause acts as a WHERE clause for groups, selecting the row groups that contribute to the summary query results.
9
Subqueries and Query Expressions
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SQL: The Complete Reference
he SQL subquery feature lets you use the results of one query as part of another query. The ability to use a query within a query was the original reason for the word structured in the name Structured Query Language. The subquery feature is less well known than SQL s join feature, but it plays an important role in SQL for three reasons: I A SQL statement with a subquery is often the most natural way to express a query, because it most closely parallels the English-language description of the query. I Subqueries make it easier to write SELECT statements, because they let you break a query down into pieces (the query and its subqueries) and then put the pieces back together. I Some queries cannot be expressed in the SQL language without using a subquery. The first several sections of this chapter describe subqueries and show how they are used in the WHERE and HAVING clauses of a SQL statement. The later sections of this chapter describe the advanced query expression capabilities that have been added to the SQL2 standard, which substantially expands the power of SQL to perform even the most complex of database operations.
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