= 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.