How Can I Efficiently Combine Multiple SELECT Statements in a Single Query?

Combining Multiple SELECT Statements Efficiently

As a common need arises in data analysis, combining multiple SELECT statements into a single, optimized query is essential. This challenge is encountered when retrieving data from multiple tables or databases that follow a similar schema, as in the example described.

To prevent the discarding of intermediate rows and retrieve a single-column result with multiple rows corresponding to each schema, the syntax of the query must be modified.

The solution lies in enclosing the individual SELECT statements within parentheses. This technique makes the syntax unambiguous and allows limit operations to be applied within the sub-statements, ensuring that only a single row is returned from each table.

Using the UNION ALL operator to combine the sub-statements creates the desired result. Here's the modified query with the necessary parentheses:

(SELECT result FROM tbl1 LIMIT 1)
UNION ALL
(SELECT result FROM tbl2 LIMIT 1)

The UNION ALL operator combines the results of the sub-statements without removing duplicates, creating a single-column result with as many rows as the number of schemas.

This technique follows the guidelines outlined in the MySQL documentation for the UNION operator, which specifies that ORDER BY and LIMIT clauses can be applied to subexpression if enclosed in parentheses. Otherwise, these clauses would apply to the overall result of the UNION and not to its individual inputs.

By utilizing the parentheses, the query ensures the efficient retrieval of data from multiple schemas, allowing for further analysis or reporting within Excel or other tools.

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