How Can I Combine Multiple SELECT Statements with LIMIT Clauses Without Losing Results?

Combining Multiple SELECT Statements without Discarding Results

In the realm of database query optimization, we often encounter the need to combine multiple SELECT statements into a single query. However, as the example provided illustrates, applying a LIMIT clause to each individual SELECT statement to restrict its result to a single row can lead to data loss.

To address this issue, we need to ensure that the individual SELECT statements are enclosed within parentheses. By doing so, we make the query syntax more unambiguous and prevent the LIMIT clause from being applied to the overall query result.

Here is how the revised query should look:

(SELECT result FROM foo.table LIMIT 1)
UNION ALL
(SELECT result FROM bar.table LIMIT 1)
UNION ALL
(SELECT result FROM doo.table LIMIT 1)

In this revised query, each individual sub-statement is enclosed within parentheses, ensuring that the LIMIT clause is applied only to its respective sub-query. As a result, all the returned rows will be preserved, providing the desired one-column result with multiple rows representing the different schema names.

It is important to note that the MySQL manual explicitly states:

"select_statement is any SELECT statement without an ORDER BY, LIMIT, FOR UPDATE, or FOR SHARE clause. (ORDER BY and LIMIT can be attached to a subexpression if it is enclosed in parentheses. Without parentheses, these clauses will be taken to apply to the result of the UNION, not to its right-hand input expression.)"

By enforcing this rule, we can ensure that our queries are executed as intended and that no data is unintentionally discarded.

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