Check if two "select"s are equivalent

sql

join

database

sql-standards

byAlex Kataev·Dec 21, 2024

The fastest route to equality checks for SELECT queries lies in the EXCEPT operator:

-- If you see no row, it's a go!
(SELECT * FROM Query1)
EXCEPT
(SELECT * FROM Query2)

UNION ALL

-- Got a row? Say 'OH NO!'
(SELECT * FROM Query2)
EXCEPT
(SELECT * FROM Query1)

If both operations yield no rows, Query1 and Query2 are congruent twins! Any result will make them more like distant cousins.

Detailed review

Key pillars of query comparison

Comparing Query1 and Query2 with EXCEPT or MINUS have a few boxes to check:

Column chronology: Columns in both queries must be in the same sequence.
Duplicate dilemma: If duplicates matter, use EXCEPT ALL.
Compatibility check: Use EXCEPT for standardized SQL; MINUS for database-specific SQL.
Complexity consideration: Peek under the bonnet of each query, especially when dealing with subqueries and joins.

Debugging discrepancies

While results comparison stands vital, remember to be on the lookout for:

Boundary bashers: Test potential edge cases that could influence exceptions.
Database dynamism: The query equivalence might vary due to current data.
Nested nuisances: Subqueries could decide whether the results match or mismatch.
Function follies: Database-specific functions and operators can swing the balance on equality.

Block-by-block breakdown

Sequencing suspects: Are ORDER BY clauses in sync in the two queries?
Limit loopholes: Do LIMIT restrictions prune results identically across queries?
Aggregation agitations: Are GROUP BY fields and particulars like COUNT() reported consistently across results?

Rigorous Testing

Ensuring equivalency effectively

Database diversity: Test across different databases for enhanced reliability.
Synthetic scenarios: Use designed test cases to validate behaviour edges.
Performance profiling: Different queries might show identical sets but differ in query efficiencies.

Future-proofing your checks

Update adherence: Validate your comparisons with changes in SQL standards or DB versions.
Schema swings: Alterations to table structures may challenge the pre-established equivalence.

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