How to Update a SQL Server Table Using a Join?

Updating SQL Server Tables via Joins: A Comprehensive Guide

This guide details how to update a SQL Server table using data from another table via a join. We'll cover the process step-by-step, illustrating with practical examples.

1. Defining the Table Relationship:

First, clearly identify the relationship between the tables involved. For example, consider tables 'sale' and 'ud'. Suppose 'sale.assid' is a foreign key referencing 'ud.assid'. This defines the link between the two tables.

2. Constructing the UPDATE Query:

The core of the update operation lies in the UPDATE statement. This statement targets the table to be modified, followed by a JOIN clause to establish the connection with the source table.

3. Specifying the Update Values:

The SET clause dictates which column(s) to update and their new values, sourced from the joined table. For instance, SET u.assid = s.assid updates 'ud.assid' with the corresponding value from 'sale.assid'.

4. Defining the Join Condition:

The JOIN clause uses the ON keyword to specify the join condition. In our example, ON u.id = s.udid links rows where 'ud.id' matches 'sale.udid'.

5. Illustrative Query:

Here's a sample query demonstrating the process:

<code class="language-sql">UPDATE ud u
SET u.assid = s.assid
FROM ud u
INNER JOIN sale s ON u.id = s.udid;</code>

This query updates 'ud.assid' for each row where a matching 'udid' exists in the 'sale' table.

Alternative Approach: Subqueries:

SQL Server also supports using subqueries within the SET clause to derive update values. This offers an alternative syntax:

<code class="language-sql">UPDATE ud u
SET u.assid = (
    SELECT s.assid
    FROM sale s
    WHERE s.udid = u.id
);</code>

Important Note: Performance and database system compatibility might influence the preferred syntax. Consider testing both approaches to determine the optimal solution for your specific scenario.

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