如何组合查询？答案

【问题标题】：how to combine queries?如何组合查询？
【发布时间】：2014-01-29 02:30:14
【问题描述】：

美好的一天。

我有一些疑问：

查询 1。

SELECT 
    n.Type as Type,
    n.UserIdn as UserIdn,
    u.Username as Username,
    n.NewsIdn as NewsIdn,
    n.Header as Header,
    n.Text as Text,
    n.Tags as Tags,
    n.ImageLink as ImageLink,
    n.VideoLink as VideoLink,
    n.DateCreate as DateCreate,
FROM News n

查询 2。

Select count(*) as Uplikes FROM Likes WHERE Type = 'up' AND NewsIdn='730456' 
// value NewsIdn for example

查询 3。

Select count(*) as DownLikes FROM Likes WHERE Type = 'down' AND NewsIdn='730456' 
// value NewsIdn for example

我会结合从查询 1 中的每个行表 News 获取 Uplikes 和 DownLikes 的查询（每个行表 News 具有唯一值 NewsIdn）。

请告诉我如何制作它？

P.S.：结果我会得到下一个 - http://clip2net.com/clip/m264191/1389339754-clip-63kb.jpg

【问题讨论】：

标签： mysql sql select count group-by

【解决方案1】：

试试这个：

SELECT 
    n.Type AS TYPE,
    n.UserIdn AS UserIdn,
    u.Username AS Username,
    n.NewsIdn AS NewsIdn,
    n.Header AS Header,
    n.Text AS TEXT,
    n.Tags AS Tags,
    n.ImageLink AS ImageLink,
    n.VideoLink AS VideoLink,
    n.DateCreate AS DateCreate, 
    SUM(l.Type = 'up') Uplikes, 
    SUM(l.Type = 'down') Downlikes
FROM News n 
INNER JOIN Users u ON n.UserIdn = u.UserIdn 
LEFT JOIN Likes l ON n.NewsIdn = l.NewsIdn
GROUP BY n.id

查看SQL FIDDLE DEMO

|  TYPE | USERIDN | USERNAME | NEWSIDN | HEADER |                      TEXT |      TAGS |      IMAGELINK | VIDEOLINK |                      DATECREATE | UPLIKES | DOWNLIKES |
|-------|---------|----------|---------|--------|---------------------------|-----------|----------------|-----------|---------------------------------|---------|-----------|
| image |  346412 |     test |  260806 |   test |                           |           | 1388152519.jpg |           | December, 27 2013 08:55:27+0000 |       2 |         0 |
| image |  108546 |    test2 |  905554 |  test2 | 1231231111111111111111111 |  123, 123 | 1388153493.jpg |           | December, 27 2013 09:11:41+0000 |       1 |         0 |
|  text |  108546 |    test2 |  270085 |  test3 |                           | 123 ,123  |                |           | December, 27 2013 09:13:30+0000 |       1 |         0 |
| image |  108546 |    test2 |  764955 |  test4 |                           |           | 1388192300.jpg |           | December, 27 2013 19:58:22+0000 |       0 |         1 |

【讨论】：

请告诉我为什么INNER JOIN Users？为什么不LEFT JOIN Users？