How Can I Efficiently Combine Single-Row Results from Multiple SELECT Queries into One Column?

Merging Multiple SELECT Queries

In a bid to extract data from numerous schemas, the user has employed Excel to generate an array of SELECT statements for a database housing an extensive number of identical schemas. Each query fetches a single result with the intention of combining them into a single column output with multiple rows representing the distinct schemas.

The initial approach involving sequential SELECT queries resulted in subsequent rows being discarded, despite the use of UNION ALL. This is because the LIMIT 1 clause restricts each subquery to a single row, impeding UNION ALL's ability to combine the results.

To alleviate this issue, parenthesizing each substatement ensures syntactic clarity and allows for proper application of LIMIT:

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

As outlined in the UNION documentation, substatements enclosed within parentheses can have ORDER BY and LIMIT clauses applied to them. By doing so, the clauses are applied to the substatement's result rather than the overall UNION operation. This enables the retrieval of a single result from each schema while combining them into a single column output.

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