How Can I Update SQL Server Tables Using SELECT Statements?

Leveraging SELECT Statements for SQL Server Table Updates

SQL Server offers flexible approaches to modifying table data. While INSERT INTO ... SELECT is standard for adding rows, you can also update existing rows using UPDATE in conjunction with a SELECT statement. This provides a powerful way to modify multiple records based on data from another table.

Here's how to perform updates using this technique:

Basic Update with SELECT:

The fundamental approach involves using the UPDATE statement to specify the target table and columns, then using a SELECT statement to provide the new values. A WHERE clause is crucial to ensure only the intended rows are modified:

<code class="language-sql">UPDATE Table SET col1 = a.col1, col2 = a.col2
FROM (SELECT col1, col2, id FROM other_table WHERE sql = 'cool') AS a
WHERE Table.id = a.id;</code>

This example updates col1 and col2 in the Table table with values from other_table, but only for rows where the id matches and the condition sql = 'cool' is met in other_table. Note the use of a subquery aliased as 'a' for clarity and efficiency.

Advanced Updates with JOINs:

For more complex scenarios involving matching rows across tables, a JOIN clause offers a cleaner and more efficient solution:

<code class="language-sql">UPDATE Table_A
SET Table_A.col1 = Table_B.col1, Table_A.col2 = Table_B.col2
FROM Some_Table AS Table_A
INNER JOIN Other_Table AS Table_B ON Table_A.id = Table_B.id
WHERE Table_A.col3 = 'cool';</code>

This UPDATE statement uses an INNER JOIN to link Some_Table and Other_Table based on matching id values. It then updates col1 and col2 in Table_A with corresponding values from Table_B only where Table_A.col3 = 'cool'. This method is generally preferred for its readability and performance, especially with larger datasets. Remember to choose the appropriate JOIN type (INNER, LEFT, RIGHT, FULL) based on your specific update requirements.

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