SQL查询中的变量

Question

In the below query I noticed these named variables wposts, wpostmeta and was wondering how they work and what are they called (MySQL variables) so I can find out more information about using them in the MySQL docs.

Is it a shorthand so you don't have to type $wpdb->postmeta every time or is there more to this? Also, I don't understand the SELECT wposts.* there is no Wordpress table called wposts so what are you selecting from?

https://codex.wordpress.org/Displaying_Posts_Using_a_Custom_Select_Query

$querystr = "
        SELECT wposts.* 
        FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
        WHERE wposts.ID = wpostmeta.post_id 
        AND wpostmeta.meta_key = 'custom-key' 
        AND wposts.post_type = 'page' 
        ORDER BY wpostmeta.meta_value DESC
 ";

Answer 1

They are table aliases, and provide (generally) a short-hand way of referring to a table in a query. In your query the table whose name was $wpdb->posts is now referred to as wposts , and the table called $wpdb->postmeta is now referred to as wpostmeta . Note that once you have declared an alias, you must refer to the table using that alias, hence in your query you have references to wposts.* , wpostmeta.meta_value etc.

Note that you can also have column aliases, for example:

SELECT SUM(x) AS total FROM t1

Also note that the AS keyword I have shown in the column select above is optional and can be used for table and column aliases.

You can read more about aliases here .