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How Can I UNION Database Tables with Different Numbers of Columns?

Posted on 2025-02-11
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How Can I UNION Database Tables with Different Numbers of Columns?

Combined tables with different columns

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Can encounter challenges when trying to merge database tables with different columns. A straightforward way is to append null values ​​to missing columns in a table with fewer columns.

For example, consider two tables, Table A and Table B, where Table A has more columns than Table B. To merge these tables and also process the missing columns in Table B, follow these steps:

  1. Determines the missing columns in Table B and adds them to the end of the table.
  2. Fill the missing columns in Table B with empty values.
  3. Use the SQL UNION operator to combine table A and modified table B.

The following SQL query demonstrates this process:

SELECT Col1, Col2, Col3, Col4, Col5
FROM Table1
UNION
SELECT Col1, Col2, Col3, NULL AS Col4, NULL AS Col5
FROM Table2;

In this query, "Col4" and "Col5" are missing columns in Table B, filled with null values ​​in the UNION clause. Therefore, the UNION operation seamlessly merges the two tables and leaves the missing column values ​​as nulls.

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