How Does SQL Operator Precedence Affect WHERE Clause Evaluation?

Understanding SQL Operator Precedence in WHERE Clauses:

Accurate SQL WHERE clause evaluation hinges on understanding operator precedence. Precedence dictates the order of evaluation for operators like AND and OR within a single expression.

AND and OR Precedence:

AND takes precedence over OR. This means AND conditions are processed before OR conditions in a combined expression. Consider these examples:

<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>

In the first query, some_col must be between 1 and 5 and satisfy some_other_expr. However, in the second query, the OR condition is evaluated after the AND condition. It checks if some_col is in (1,2,3) or (if that's false) if some_col is in (4,5) and some_other_expr is true.

Truth Table Illustration:

The following truth table highlights the difference in evaluation:

some_col	some_other_expr	First Statement	Second Statement
1	True	True	True
1	False	False	False
3	True	True	True
3	False	False	False
4	True	False	True
4	False	False	False
5	True	False	True
5	False	False	False

Notice the differing results when some_col is 4 or 5. This discrepancy stems from the precedence of AND over OR.

Controlling Precedence with Parentheses:

To explicitly control the order of operations, use parentheses to group conditions. This overrides the default precedence. For example:

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

Here, the OR conditions are evaluated first, then the AND condition is applied to the result.

Further Reading:

For detailed information on operator precedence, consult the documentation for your specific database system:

• Microsoft SQL Server Transact-SQL operator precedence
• MySQL operator precedence
• Oracle operator precedence
• PostgreSQL operator precedence
• SQLite operator precedence

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