How Does SQL's Logical Operator Precedence Affect Query Results?

SQL logical operator precedence: understanding its importance

In SQL, the order of operations of logical operators (especially "AND" and "OR") plays a crucial role in determining query results. Operators with higher precedence are evaluated before operators with lower precedence, which may lead to different results.

AND and OR priority

In SQL, "AND" has a higher priority than "OR". Consider the following example:

<code class="language-sql">SELECT [...]
FROM [...]
WHERE some_col in (1,2,3,4,5) AND some_other_expr</code>

<code class="language-sql">SELECT [...]
FROM [...]
WHERE some_col in (1,2,3) or some_col in (4,5) AND some_other_expr</code>

The first statement returns rows where some_col is in the range 1-5 and some_other_expr is true. However, since "AND" has higher precedence than "OR", the second statement is not equivalent.

Evaluation of the second statement

Since "AND" has higher precedence, the second statement is calculated as follows:

<code class="language-sql">WHERE (some_col in (1,2,3) or some_col in (4,5)) AND some_other_expr</code>

This means that the query returns rows with:

• some_col in the range of 1-3, or
• some_colIn the 4-5 range

then

• some_other_expr is true

To achieve the intended functionality, the priorities can be overridden using parentheses:

<code class="language-sql">WHERE (some_col in (1,2,3) OR some_col in (4,5)) AND some_other_expr</code>

This ensures that the query returns rows with:

• some_colIn the range of 1-5

then

• some_other_expr is true

Priority Reference

For those seeking further clarification, please refer to the following resources:

• Microsoft Transact-SQL operator precedence
• Oracle MySQL 9 operator precedence
• Oracle 10g condition priority
• PostgreSQL operator precedence
• SQL understood by SQLite

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