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:

SELECT [...]
FROM [...]
WHERE some_col IN (1,2,3,4,5) AND some_other_expr

SELECT [...]
FROM [...]
WHERE some_col IN (1,2,3) OR some_col IN (4,5) AND some_other_expr

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:

SELECT [...]
FROM [...]
WHERE (some_col IN (1,2,3) OR some_col IN (4,5)) AND some_other_expr

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

The above is the detailed content of How Does SQL Operator Precedence Affect WHERE Clause Evaluation?. For more information, please follow other related articles on the PHP Chinese website!