How Does Operator Precedence Affect MySQL Query Results with AND and OR?

Preponderance of Boolean Operators in MySQL

Queries often leverage logical operators like OR and AND to filter and retrieve specific data. Understanding the precedence of these operators is crucial for ensuring queries execute as intended.

In MySQL, operator precedence determines the order in which operations are evaluated. The MySQL documentation provides a comprehensive list of operator precedence levels, with operators at the top of the list having higher precedence.

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 LIKE "%hello world%";

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

1. Parentheses first:

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

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2. Left-to-right evaluation:

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

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To avoid ambiguity, it's best practice to use parentheses to group expressions and explicitly define operator precedence. For example:

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

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