掌握 WP_Meta_Query 和 WP_Date_Query
本文是《掌握 WP_Query》专题的第 17 篇，共 19 篇：
- WP_Query 参数：自定义字段（Custom Fields）
- WP_Query 参数：分类法（Taxonomies）
- 掌握 WP_Query : 入门介绍
- 掌握 WP_Query：教你使用Loop循环
- 掌握 WP_Query：相关的函数
- 掌握 WP_Query：行动器和过滤器
- 掌握 WP_Query：WP_Query类的属性和方法
- WP_Query 参数：文章、页面和文章类型
- WP_Query 参数：分类和标签
- WP_Query 参数：日期
- WP_Query 参数：状态、排序和分页
- WP_Query 参数：作者、搜索、密码、权限、缓存和返回字段
- 掌握 WP_Query：10个有用的例子
- 结合 WP_Query 与主查询（the Main Query）
- 掌握 WP_User_Query
- 掌握 WP_Comment_Query
- 掌握 WP_Meta_Query 和 WP_Date_Query
- WordPress 4.1的查询改进
- 掌握 WP_Query：结尾
欢迎来到该系类的最后一个部分 —— 好吧，你猜对了，技术上来说最后一个部分应该是“系列结尾”。在这个部分，我们来学习同级类 WP_Meta_Query 和 WP_Date_Query。
Welcome to the final part of the series—well, technically the final part will be "Series Finale", but you get the idea. In this part, you're going to learn about two sibling classes called
Without further ado, let's begin!
WP_Meta_Query class is a "helper class", helping
WP_Query to make queries with meta data.
As you know, WordPress stores three kinds of meta data in the database: post meta, user meta and comment meta. We saw in the previous tutorials that we can run meta queries within the queries that we make with the
WP_Comment_Query classes (with the
'meta_query' parameter). The
WP_Meta_Query is actually running when you do those queries.
Turns out, you can get the SQLs for these meta-related queries with the help of the
WP_Meta_Query class. This class doesn't really get the results of the given query, but instead prepares the SQL commands for you to use somewhere else.
We can't say it's a tutorial if we don't do an example, right? With a simple example, we're going to see how we can use the
WP_Meta_Query class in real life. (Granted, it's an extremely specific thing to get the SQL code for a meta-related query, but I will try to come up with a real-world example.)
Let's imagine that you want to make a special "related posts plugin" for your own website, where you will list posts which have the same meta value for a meta key—or another meta value for another meta key. And instead of making a meta query within a
WP_Query instance, you want to get the SQL code of the query to use it dynamically in separate code blocks. Here are the steps to prepare that SQL code:
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<?php global $wpdb; $my_meta_query_args = array( 'relation' => 'OR', array( 'meta_key' => 'Some_Key', 'meta_value' => 'Some_Value', 'compare' => '=' ), array( 'meta_key' => 'Some_Other_Key', 'meta_value' => 'Some_Other_Value', 'compare' => '=' ) ); $my_meta_query = new WP_Meta_Query; $my_meta_query->parse_query_vars( $my_meta_query_args ); $my_meta_query_sql = $my_meta_query->get_sql( 'post', $wpdb->posts, 'ID' ); ?>
There you go: The
$my_meta_sql variable stores the SQL code for your special query, and you can use this SQL code anywhere you like within your project.
WP_Date_Query is a helper class for the
WP_Comment_Queryclasses. This helper class was introduced in WordPress version 3.7. Back then, the class didn't support
WP_User_Query, but since the 4.1 version, you can query inside the users table (the
user_registered column specifically).
WP_Meta_Query and its ability to query meta keys and values, the
WP_Date_Query class allows us to query date fields inside the posts, comments and users tables. And exactly like
WP_Meta_Query, this helper class also lets you return the prepared SQL code to run a date-related query.
In order to fully understand how the
WP_Date_Query class works, let's go through an example with it. It's going to be yet another unnecessarily specific example, but it wouldn't feel right to leave this part without an example.
Let's imagine that, for some reason, we need to query for comments that are made in the current month and before noon. (Please shoot me a comment if you find a good case to fetch comments made in the current month and before noon!) Here's how to get the SQL code for this bizarre query:
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<?php $my_date_query_args = array( array( 'month' => date( 'n' ), ), array( 'before' => 'noon' ), 'relation' => 'AND' ); $my_date_query = new WP_Date_Query( $my_date_query_args, 'comment_date' ); $my_date_query_sql = $my_date_query->get_sql(); ?>
There you go. Keep in mind that you can use PHP relative date formats, which are really useful.
Quick tip: Christian Bruckner has a great post on MarketPress.com about how
WP_Date_Query works. It's a little bit outdated (because it was written before WordPress 4.1 was released) but it's very well-written and still a good read. Be sure to check it out.
With these two helper classes, we're ending the long journey of dissecting the
WP_Query class. This was one of the longest tutorial series in the history of Tuts+, so thank you for bearing with us till the end! In the next (and last) part, we're going to recap what we went through for the last time and close the series.
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