The MySQL documentation has a good page with information on which operators take precedence.
From that page,
12.3.1. Operator Precedence
Operator precedences are shown in the following list, from highest precedence to the lowest. Operators that
are shown together on a line have the same 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), :=
So your original query
Select
*
from tablename
where
display = 1
or display = 2
and content like "%hello world%"
or tags like "%hello world%"
or title = "%hello world%"
would be interpreted as
Select
*
from tablename
where
(display = 1)
or (
(display = 2)
and (content like "%hello world%")
)
or (tags like "%hello world%")
or (title = "%hello world%")
When in doubt, use parenthesis to make your intent clear. While the information on the MySQL page is helpful, it may not be immediately obvious if the query is ever revisited.
You might consider something like the following. Note that I’ve changed the title = "%hello world%"
to title like "%hello world%"
, since that fits better with the goal you’ve described.
Select
*
from tablename
where
(
(display = 1)
or (display = 2)
) and (
(content like "%hello world%")
or (tags like "%hello world%")
or (title like "%hello world%")
)