Sql INSERT INTO from multiple tables

Preserving data integrity

Ensure compatibility of data types and constraints between the source and destination tables prior to executing the INSERT INTO operation.

A helpful practice is to envelop the operation within a transaction, ensuring a successful operation or a fresh restart in case of an error:

BEGIN;
-- Moment of silence for the operations that don't make it...
INSERT INTO Table3 (colA, colB)
SELECT t1.colX, t2.colY
FROM Table1 t1
JOIN Table2 t2 ON t1.id = t2.t1_id;
COMMIT;
-- If you've arrived here, congratulations! Your operation was a success! 
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Beyond the basic "id" join

In various scenarios, you might want to grab all possible record combinations from your tables. FULL JOINS lets you do exactly this:

INSERT INTO Table3 (colA, colB)
SELECT t1.colX, t2.colY
FROM Table1 t1
FULL JOIN Table2 t2 ON t1.id = t2.t1_id;
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A friendly reminder: FULL JOINS can result in NULLs, handle them accordingly.

Improving performance and indexing

Merging large datasets warrants indexing on the JOIN key columns. Proper Indexing can speed up query performance during JOIN operations.

Suppose if Table1.id and Table2.t1_id are frequently used for JOIN operations, create indexes on these columns:

CREATE INDEX idx_t1_id ON Table1 (id);
CREATE INDEX idx_t2_t1_id ON Table2 (t1_id);
-- SQL indexing, now that's 'fast food'. 


Test drive

Before handing off the INSERT INTO command to work on your entire dataset, test it with a small subset of data to ensure efficacy. This can help prevent any catastrophic errors during a significant data operation.

Navigating complexities

For complex scenarios or encountered errors, do not hesitate to seek aid from the community or experts. Validation and guidance can go a long way in ensuring a successful execution of your data manipulation.