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MySQL OR/AND Precedence
MySQL's logical operators have specific precedence rules that dictate how multiple conditions are evaluated in a query. Understanding these rules is crucial for crafting efficient and accurate queries.
Regarding your query, you want to retrieve rows where display is 1 or 2 and where any of content, tags, or title contains "hello world." The precedence rules for MySQL operators are:
(highest precedence) INTERVAL BINARY, COLLATE ! - (unary minus), ~ (unary bit inversion) ^ *, /, DIV, %, MOD -, + <<, >> & | = (comparison), <=, >=, >, <, <>, !=, IS, LIKE, REGEXP, IN BETWEEN, CASE, WHEN, THEN, ELSE NOT &&, AND XOR ||, OR = (assignment), := (lowest precedence)
With these rules in mind, let's analyze your 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 precedence rules, the logical operators are evaluated within parentheses:
Due to the precedence of OR over AND, the query will be interpreted as follows:
Select * from tablename where (display = 1 or display = 2) and (content like "%hello world%" or (tags like "%hello world%" or title = "%hello world%"))
Therefore, the query will retrieve rows where:
To avoid confusion, it's recommended to use parentheses to explicitly indicate the desired evaluation order. For instance, the following query will retrieve rows where either display is 1 or 2, or where any of content, tags, or title contains "hello world":
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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