Which Approach is Best for Selecting Rows Present in One Table But Not Another: \'NOT EXISTS\' or \'LEFT JOIN\'?

Optimizing "Mysql select where not in table" for Efficient Data Retrieval

When dealing with two tables (A and B) with identical primary keys, the task of selecting rows present in A but absent in B arises commonly. The standard approach, using the "NOT EXISTS" clause, suffers from performance limitations.

Not Exists vs. Left Join

The "NOT EXISTS" query you mentioned checks for the absence of matching rows in B for each row in A. This process, known as an anti-join, can be inefficient for large datasets. In contrast, your suggested left join approach looks for null values in the B.y column for each row in A. This can be a more effective solution.

Improved Query Expression

However, there is a minor improvement that can be made to your left join query:

SELECT A.*    
FROM A LEFT JOIN B ON A.x = B.y
WHERE B.y IS NULL

By explicitly selecting only columns from table A (A.*), you optimize the query by retrieving only the necessary data.

Conclusion

While the "NOT EXISTS" clause remains a viable option for small datasets, the left join approach with the improved query expression provides better performance for larger data volumes. This allows you to efficiently retrieve rows that exist in table A but not in table B.

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