How Does MySQL Handle OR and AND Operator Precedence in WHERE Clauses?

MySQL OR/AND Operator Precedence Explained

Understanding operator precedence is crucial for interpreting the logical flow of SQL queries. In MySQL, OR and AND have differing precedences, affecting the order in which they are evaluated.

Query Interpretation

Let's consider the following query:

SELECT * FROM tablename
WHERE display = 1 OR display = 2
AND content LIKE "%hello world%"
OR tags LIKE "%hello world%"
OR title = "%hello world%"

According to the MySQL documentation, OR and AND have equal precedence. Therefore, the query can be interpreted in two ways:

Interpretation 1:

(display = 1 OR display = 2) AND (content LIKE "%hello world%" OR tags LIKE "%hello world%" OR title = "%hello world%")

This interpretation prioritizes the OR operator within parentheses, grouping rows where display equals 1 or 2 with rows matching any of the content, tags, or title criteria.

Interpretation 2:

((display = 1 OR display = 2) AND (content LIKE "%hello world%")) OR (tags LIKE "%hello world%" OR title = "%hello world%")

In this interpretation, the AND operator binds more tightly than OR. Display values must both match 1 or 2, AND at least one of the content, tags, or title conditions must be met.

Precedence Rules

To avoid confusion, use the following operators precedence rules:

1. Parentheses always override precedence.
2. Operators with the same precedence are evaluated from left to right.
3. AND has higher precedence than OR.

Recommended Approach

For clarity, explicitly group clauses using parentheses, especially when using multiple logical operators. Consider the following modified query:

SELECT * FROM tablename
WHERE
    (display = 1 OR display = 2)
    AND (content LIKE "%hello world%" OR tags LIKE "%hello world%" OR title LIKE "%hello world%")

This approach clearly defines the intended logical flow, ensuring the query behaves as expected.

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