In sql, which one is executed first, where or on?

The execution order of WHERE and ON clauses is: 1. The WHERE clause is executed first and rows that meet the conditions are filtered out. 2. The ON clause is then applied to the filtered rows, establishing a join based on the join conditions.

Execution order of WHERE and ON clauses in SQL

In SQL query, WHERE and ON clauses is an important structure for filtering data sets. Understanding the order in which these two clauses are executed is critical to optimizing query performance.

Execution order:

The WHERE clause is executed before the ON clause.

Detailed explanation:

• The WHERE clause is applied to the entire table in the query to filter out rows that meet the specified conditions.
• The ON clause is used to join tables and specify the join conditions, that is, which rows should match.
• Since the WHERE clause is applied before the ON clause, it first filters out the rows that meet its condition.
• Then, the ON clause performs a join operation on the filtered rows and establishes a join based on the rows that meet its join conditions.

Example:

<code class="sql">SELECT *
FROM table1
WHERE column1 = 'value1'
INNER JOIN table2 ON table1.column2 = table2.column3;</code>

In this query, the WHERE clause first filters out column1 in table1 Rows equal to value1. The ON clause then joins table1 and table2 if table1.column2 is equal to table2.column3. Only rows that satisfy both the WHERE and ON conditions will be returned.

The importance of understanding the order of execution:

• Understanding the order of execution of WHERE and ON clauses can help prevent unexpected results in queries.
• You can improve the efficiency of join operations by ensuring that the WHERE clause filters out fewer data rows.
• Optimizing the execution order can be achieved by using indexes in the WHERE clause and restricting the join conditions of the ON clause.

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