mysql查询从多个中选择，按日期排序

Question

What's a good and simple way to select from multiple tables and just order everything by date, newest to oldest?

 mysql_query("
               SELECT * 
               FROM posts, comments, photos 
               WHERE userID='$session' 
               ORDER BY date");

if I wanted to do something like that.

Answer 1

SELECT *
FROM (SELECT userID, Col1, Col2, Col3, date
      FROM posts
      UNION
      SELECT userID, Col1, Col2, Col3, date
      FROM comments
      SELECT userID, Col1, Col2, Col3, date
      FROM photos ) t
WHERE userID = 123
ORDER BY date DESC

Answer 2

Your existing query won't do what you want. Instead you need to JOIN your tables on some common column:

SELECT
  posts.*,
  comments.*,
  photos.*
FROM 
  posts JOIN comments ON posts.post_id = comments.post_id
  JOIN photos ON posts.post_id = photos.post_id
ORDER BY posts.date DESC

Note that it is often not advisable to do SELECT * or SELECT posts.* in production code. It is typically better to explicitly list the columns you want so you can be sure of the order they arrive in (which matters for UNION queries, for example), and if your schema changes.

SELECT
  posts.date,
  posts.post_id,
  posts.title,
  posts.etc,
  comments.date,
  comments.user,
  comments.text,
  photos.title
FROM
  posts JOIN comments ON posts.post_id = comments.post_id
  JOIN photos ON posts.post_id = photos.post_id

/* Also, if multiple tables have a `date` column, you'll need to specify which one
   as in `posts`.`date` */
ORDER BY posts.date DESC

Answer 3

To use an ORDER BY or LIMIT clause to sort or limit the entire UNION result, parenthesize the individual SELECT statements and place the ORDER BY or LIMIT after the last one. The following example uses both clauses:

(SELECT a FROM t1 WHERE a=10 AND B=1) UNION 
(SELECT a FROM t2 WHERE a=11 AND B=2) 
ORDER BY a LIMIT 10;

A statement without parentheses is equivalent to one parenthesized as just shown.

This kind of ORDER BY cannot use column references that include a table name (that is, names in tbl_name.col_name format). Instead, provide a column alias in the first SELECT statement and refer to the alias in the ORDER BY. (Alternatively, refer to the column in the ORDER BY using its column position. However, use of column positions is deprecated.)

-> http://dev.mysql.com/doc/refman/5.0/en/union.html