Mysql or/and precedence?

Question

I am wondering how or/and works?

For example if I want to get all rows where display = 1

I can just do WHERE tablename.display = 1

and if I want all rows where display = 1 or 2

I can just do WHERE tablename.display = 1 or tablename.display = 2

But what if I want to get all rows where display = 1 or 2 and where any of the content, tags, or title contains hello world

How would the logic play out for that?

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 my guess. but then I can read that in several ways.

Does it read out as:

 (display = 1 or display = 2) and (content like "%hello world%" or tags like "%hello world%" or title = "%hello world%")

or as

((display = 1 or display = 2) and (content like "%hello world%")) or (tags like "%hello world%" or title = "%hello world%")

etc.

Solution 1

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%")
    )

Solution 2

Run this query:

select 1 or 1 and 0

If it comes out as 1, then that means the precedence is:

select 1 or (1 and 0)

if it comes out 0, then the precedence is:

select (1 or 1) and 0

Spoiler: it comes out 1

That is to say, ANDs are evaluated before ORs, or as I like to say, ANDs are stickier.

Solution 3

You need to use brackets for your multiple OR conditions. And for display = 1 OR display = 2 you can use display IN(1,2). Try this:

SELECT * FROM tableName
WHERE display IN (1,2)
AND (content LIKE "%hello world%" 
OR tags LIKE "%hello world%" 
OR title LIKE "%hello world%")

For more info look at MySQL: Operator Precedence

Comments

[zerkms]

dev.mysql.com/doc/refman/5.5/en/operator-precedence.html

[Salman A]

These things are taught during your early C/C++ classes.

[Hailwood]

Then I guess I should sign up for an entire C/C++ course just to learn the operator precedence :/

[Hailwood]

Please see my addition to my question. it's in relation to not having brackets how does it play out?