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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.



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up vote 29 down vote accepted

The first several work about like 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 are SQL operators that let you add in wild-card symbols, so you could have a meta query that looks like this:

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

    '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 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:

    '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 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:

    'key' => 'price', 
    'compare' => 'NOT EXISTS'

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

Hope this helps!

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Beat me to it. So, +1 for the explanation! – userabuser Oct 29 '12 at 18:50
lol userabuser, you were editing the original question! thanks for the upvote! – guiniveretoo Oct 29 '12 at 18:54
Thanks a lot!! This is exactly what I needed! – dev-jim Oct 29 '12 at 18:55
Note: If you're using meta_query array, your keys should not be prefixed with meta_. If you're using $query->meta_key, $query->meta_value, etc. then these should still retain the prefix. – Sean Aug 20 '14 at 14:25
@Joe, I don't know why I didn't add anything about "IN" and "NOT IN". I've edited and updated the answer with those comparisons. – guiniveretoo Mar 9 '15 at 16:05

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.

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is there info about that in the docs somewhere Herb? Should the example change to remove the %? – guiniveretoo Nov 22 '13 at 15:47
It should, I actually did that right now, see the source, then it gets very clear that Herb is right. @guiniveretoo – ialocin Apr 8 '14 at 15:10

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