MYSQL枚举:@rownum,奇数和偶数记录

Question

我问了一个关于为查询结果创建临时/虚拟ID的问题, mysql和php:查询结果的临时/虚拟ID？

我几乎得到了这个链接, http://craftycodeblog.com/2010/09/13/rownum-simulation-with-mysql/

我设法列出每一行,

SELECT 
u.pg_id AS ID, 
u.pg_url AS URL,
u.pg_title AS Title,
u.pg_content_1 AS Content,
@rownum:=@rownum+1 AS rownum

FROM (
    SELECT pg_id, pg_url,pg_title,pg_content_1
    FROM root_pages

    WHERE root_pages.parent_id = '7'
    AND root_pages.pg_id != '7'
    AND root_pages.pg_cat_id = '2'
    AND root_pages.pg_hide != '1'

    ORDER BY pg_created DESC
) u,

(SELECT @rownum:=0) r

结果,

ID  URL     Title   Content     rownum
53  a       x       x           1
52  b       x       x           2
43  c       x       x           3
41  d       x       x           4

但我怎么能更进一步 - 我想显示奇数或偶数记录只有下面的那些 - 是否可能？

奇怪的记录,

ID  URL     Title   Content     rownum
53  a       x       x           1
43  c       x       x           3

甚至记录,

ID  URL     Title   Content     rownum
52  b       x       x           2
41  d       x       x           4

谢谢.

ps我实际上并不太了解sql查询,即使我几乎得到了答案,例如,'u'和't'是什么意思？

Answer 1

'你'和't'是什么意思？

它们是表别名,因此您无需在需要引用时指定表的完整名称.

要仅获取奇数记录,请使用:

SELECT x.*
  FROM (SELECT u.pg_id AS ID, 
               u.pg_url AS URL,
               u.pg_title AS Title,
               u.pg_content_1 AS Content,
               @rownum := @rownum + 1 AS rownum
          FROM root_pages u
          JOIN (SELECT @rownum := 0) r
         WHERE u.parent_id = '7'
           AND u.pg_id != '7'
           AND u.pg_cat_id = '2'
           AND u.pg_hide != '1'
      ORDER BY u.pg_created DESC) x
WHERE x.rownum % 2 != 0

要获取偶数编号的记录,请使用:

SELECT x.*
  FROM (SELECT u.pg_id AS ID, 
               u.pg_url AS URL,
               u.pg_title AS Title,
               u.pg_content_1 AS Content,
               @rownum := @rownum + 1 AS rownum
          FROM root_pages u
          JOIN (SELECT @rownum := 0) r
         WHERE u.parent_id = '7'
           AND u.pg_id != '7'
           AND u.pg_cat_id = '2'
           AND u.pg_hide != '1'
      ORDER BY u.pg_created DESC) x
WHERE x.rownum % 2 = 0

说明

它%是MySQL语法中的模数运算符 - 它返回除法的余数.例如,1%2是0.5,而2%2是零.然后在WHERE子句中使用它来过滤显示的行.