Meta_query compare operator explanation

Question

I noticed that there are bunch of operator can be use for compare in meta_query. However, I am not quite sure what operator I should use, it is somehow confusing like = and LIKE operator.

I would like to know what exactly each operator mean, and in what condition I should use them.

=
!=
>
>=
<
<=
LIKE
NOT LIKE
IN
NOT IN
BETWEEN
NOT BETWEEN
NOT EXISTS

Thanks.

Answer 1

The first several work as you would expect:

=   equals
!=  does not equal
>   greater than
>=  greater than or equal to
<   less than
<=  less than or equal to

---

LIKE and NOT LIKE

LIKE and NOT LIKE are SQL operators that let you add in wild-card symbols, so you could have a meta query that looks like this:

array( 
    'key' => 'name', 
    'value' => 'Pat', 
    'compare' => 'LIKE'
)

This would return all posts where the meta value "name" has the string "Pat". In this case, "Pat" "Patricia" and "Patrick" would all be returned back to you. There's a non-WordPress tutorial explanation here.

Adding the wildcard character % isn't necessary, because it gets added by default like @Herb said in his below answer. Like this: $meta_value = '%' . like_escape( $meta_value ) . '%'; - see source.

---

IN and NOT IN

IN and NOT IN select any matches that are in (or not in) the given array. So you could do something like this:

array(
    'key'     => 'color', 
    'value'   => array('red', 'green', 'blue') 
    'compare' => 'IN'
)

and it would get all posts that have the color set to either red, green, or blue. Using 'NOT IN' gets the reverse, any posts that have a value set to anything else than what's in the array.

The generated SQL for this would look something like this:

SELECT * FROM posts_meta WHERE value IN ("red", "green", "blue") 

---

BETWEEN and NOT BETWEEN

BETWEEN and NOT BETWEEN allow you to define a range of values that could be correct, and require you to give two values in an array in your meta_query:

array( 
    'key' => 'price', 
    'value' => array(20,30) 
    'compare' => 'BETWEEN'
)

This will get you all posts where the price is between 20 and 30. This person digs into an example with dates.

---

NOT EXISTS

NOT EXISTS is just like what it sounds - the meta value isn't set or is set to a null value. All you need for that query is the key and comparison operator:

array( 
    'key' => 'price', 
    'compare' => 'NOT EXISTS'
)

This person needed to query non-existent meta values, and needed them to play nice with others.

Hope this helps!

Answer 2

Note that when using a meta_compare value of 'LIKE', WordPress automatically wraps the wildcard character ( % ) around the meta_value string. So the 'Pat%' example could fail to return any results.