THEN statement in a SQL query - mysql

Question

I'm trying to alter a Wordpress search query so that it returns broader results.
The scenario:
There is a post entry titles "Carousel Watch" in the database. When I search either (or both) of these words, I get the post returned, which is what I'd expect.
If, however, I search for "Carousel Watch Gift", I do not get any results.
The SQL query for this search is the following:
SELECT SQL_CALC_FOUND_ROWS sosen_posts.ID FROM sosen_posts WHERE 1=1
AND (((sosen_posts.post_title LIKE '%carousel%') OR (sosen_posts.post_content LIKE '%carousel%'))
AND ((sosen_posts.post_title LIKE '%watch%') OR (sosen_posts.post_content LIKE '%watch%'))
AND ((sosen_posts.post_title LIKE '%gift%') OR (sosen_posts.post_content LIKE '%gift%')))
AND (sosen_posts.post_password = '') AND sosen_posts.post_type = 'wp_aff_products' AND (sosen_posts.post_status = 'publish')
ORDER BY
(CASE WHEN sosen_posts.post_title LIKE '%carousel watch gift%'
THEN 1
WHEN sosen_posts.post_title LIKE '%carousel%' AND sosen_posts.post_title LIKE '%watch%' AND sosen_posts.post_title LIKE '%gift%'
THEN 2
WHEN sosen_posts.post_title LIKE '%carousel%' OR sosen_posts.post_title LIKE '%watch%' OR sosen_posts.post_title LIKE '%gift%'
THEN 3
WHEN sosen_posts.post_content LIKE '%carousel watch gift%'
THEN 4 ELSE 5 END), sosen_posts.post_date DESC LIMIT 0, 3
My questions are:
What part of the query prevents returning results ("Carosuel Watch")?
How I should alter it to get the result returned?
And finally, what does the THEN statement do in this query? Does it set the value to compare in the WHERE statement?

Answer 1

The THEN keyword is used as part of a CASE clause. A CASE clause looks like this:
CASE WHEN expression THEN value WHEN otherexpression THEN othervalue ... END
The result of the expression is a single value (sometimes you see people try to use a CASE clause to determine what code will execute, and this won't work).
In this query, the value from the CASE claues is used to determine sort order for posts, and has not bearing on which records are included or not included in the results.
To find out which records are included in the results, we need to dissect the WHERE clause. I re-formatted the existing clause for easier reading below:
WHERE 1=1 AND (
(sosen_posts.post_title LIKE '%carousel%' OR sosen_posts.post_content LIKE '%carousel%')
AND (sosen_posts.post_title LIKE '%watch%' OR sosen_posts.post_content LIKE '%watch%')
AND (sosen_posts.post_title LIKE '%gift%' OR sosen_posts.post_content LIKE '%gift%')
) AND sosen_posts.post_password = '' AND sosen_posts.post_type = 'wp_aff_products' AND sosen_posts.post_status = 'publish'
First, the 1=1 part. This is common for autogenerated code. The generator can put a WHERE 1=1 at the beginning of the WHERE clause whether or not there are any conditions, and the query will still be valid. Then, for each condition, it can always use the form AND condition, without worrying about the prior state of the WHERE clause.
Moving into the next section, we see it checks each keyword individually against both the title and text. These checks are connected via AND operators. This means that if your post does not have the word gift in the title or body somewhere, it cannnot appear in the results.
Finally, this code is incredibly inefficient. You never want to see a LIKE operator with a leading wildcard (%), because it pretty much guarantees that you can't use any indexes to satisfy that condition. I'm more of a Sql Server guy, but in Sql Server -land what you want to do instead is create a special kind of index, called a full text index, and write the query using a special CONTAINS() clause. Failing that, you use a 3rd-party search library such as Lucene. You never want to use LIKE queries for searches like this. I'm not sure what the MySql equivalents to these alternatives are, but what you have here is not the way you want to be doing this.

Answer 2

The LIKE clauses require one or more of each of the input words to exist (I suspect this is how the auto-generator works). If your posts do not contain all 3 words it will not therefore get returned.
You can remove the LIKE clauses for "Gift", though I think this will give you the same code as if you just searched for the first two words.
THEN comes before what is to be returned by the CASE function IE WHEN {true} THEN {return_this}
Hope this helps.

Answer 3

giving your query a nicer format (see below) it results that in the where clause you are filtering rows that have carousel in the title or content, watch AND gift.
The case statement only changes the ordering of the result, but if you build your query with those three words it will fetch rows with all those words.
SELECT SQL_CALC_FOUND_ROWS sosen_posts.ID FROM sosen_posts
WHERE 1=1
AND (((sosen_posts.post_title LIKE '%carousel%')
OR (sosen_posts.post_content LIKE '%carousel%')
)
AND ((sosen_posts.post_title LIKE '%watch%')
OR (sosen_posts.post_content LIKE '%watch%')
)
AND ((sosen_posts.post_title LIKE '%gift%')
OR (sosen_posts.post_content LIKE '%gift%')
)
)
AND (sosen_posts.post_password = '')
AND sosen_posts.post_type = 'wp_aff_products'
AND (sosen_posts.post_status = 'publish')
ORDER BY (CASE WHEN sosen_posts.post_title LIKE '%carousel watch gift%' THEN 1
WHEN sosen_posts.post_title LIKE '%carousel%' AND sosen_posts.post_title LIKE '%watch%' AND sosen_posts.post_title LIKE '%gift%' THEN 2
WHEN sosen_posts.post_title LIKE '%carousel%' OR sosen_posts.post_title LIKE '%watch%' OR sosen_posts.post_title LIKE '%gift%' THEN 3
WHEN sosen_posts.post_content LIKE '%carousel watch gift%' THEN 4
ELSE 5
END),
sosen_posts.post_date DESC
LIMIT 0, 3
Also note that LIMIT 0 will return an empty set.