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

Merge tables with different number of columns

Challenges may be encountered when trying to merge database tables with different number of columns. A straightforward approach is to append null values ​​to missing columns in tables with fewer columns.

For example, consider two tables, Table A and Table B, where Table A has more columns than Table B. In order to merge these tables while handling the missing columns in table B, follow these steps:

1. Identify the missing columns in table B and add them to the end of the table.
2. Fill missing columns in table B with null values.
3. Use the SQL UNION operator to combine table A and modified table B.

The following SQL query demonstrates this process:

<code class="language-sql">SELECT Col1, Col2, Col3, Col4, Col5
FROM Table1
UNION
SELECT Col1, Col2, Col3, NULL AS Col4, NULL AS Col5
FROM Table2;</code>

In this query, "Col4" and "Col5" are missing columns in table B and are filled with null values ​​in the UNION clause. Therefore, the UNION operation will merge the two tables seamlessly and leave missing column values ​​as null.

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