JOIN vs. WHERE Clauses: Does Condition Placement Impact Query Performance?

Conditional placement in JOIN and WHERE clauses: Performance and best practices

Relational algebra allows placing conditions in a JOIN or WHERE clause. This interchangeability begs the question: Are there significant differences in performance or best practices between the two approaches?

Let’s check these two examples:

<code class="language-sql">-- 条件在 JOIN 中
SELECT *
FROM dbo.Customers AS CUS
INNER JOIN dbo.Orders AS ORD
ON CUS.CustomerID = ORD.CustomerID
AND CUS.FirstName = 'John';

-- 条件在 WHERE 中
SELECT *
FROM dbo.Customers AS CUS
INNER JOIN dbo.Orders AS ORD
ON CUS.CustomerID = ORD.CustomerID
WHERE CUS.FirstName = 'John';</code>

According to relational algebra, these queries should produce the same results. However, the optimizer may rearrange the predicates in the WHERE clause so that they are applied during the JOIN procedure, potentially improving efficiency.

The recommended approach is to prioritize readability when writing queries. In some cases, placing certain conditions in a WHERE clause can enhance maintainability, for example:

<code class="language-sql">SELECT *
FROM Customers c
INNER JOIN CustomerAccounts ca
ON ca.CustomerID = c.CustomerID
WHERE c.State = 'NY'
AND ca.Status = 1;</code>

This query is more readable than putting all the conditions in a JOIN clause. However, the optimal location depends on the specific scenario and optimization goals. Performance tuning tools and analysis techniques can be used to determine the most effective approach.

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