[TOC]

1 where 语句中实现 case when

case

WHERE

pw='correct'

AND CASE WHEN id<800 THEN success=1 ELSE TRUE END

AND YEAR(timestamp)=2011

goals：return success=1 (which can be TRUE or FALSE) in case id < 800, or always return TRUE otherwise.

solution

where

pw = 'correct'

and (id > 800 or success = 1)

and year(timestamp)=2011

2 order by before group by

case

SELECT wp_posts.* FROM wp_posts

WHERE wp_posts.post_status='publish'

AND wp_posts.post_type='post'

GROUP BY wp_posts.post_author

ORDER BY wp_posts.post_date DESC

goals：The example query produces unusable results as its not always the latest post that is returned.

solution

subquery：

SELECT wp_posts.* FROM

(

SELECT *

FROM wp_posts

ORDER BY wp_posts.post_date DESC

) AS wp_posts

WHERE wp_posts.post_status='publish'

AND wp_posts.post_type='post'

GROUP BY wp_posts.post_author

join(the best solution!)：

select * from `wp_posts`

inner join

(

select max(post_date) as `max_post_date`, post_author

from `wp_posts`

where wp_posts.post_status='publish'

AND wp_posts.post_type='post'

group by wp_post.post_author

) as p

on wp_posts.post_author = p.post_author

and wp_posts.post_date = p.max_post_date

where wp_posts.post_status='publish'

AND wp_posts.post_type='post'

order by wp_posts.post_date desc

3 COALESCE() 函数

此函数返回参数中第一个不为 null 参数的值

case

goals：取出系统分、专家评分，若专家分不为 null 则以专家评分为准

solution

coalesce(sme_score, sys_score, 0)

4 load 命令

load 在大批量的导入和导出数据时效率非常高

LOAD DATA LOCAL INFILE 'C:/Users/hkf/Desktop/task_ningxia.txt' REPLACE INTO TABLE `task_ningxia` FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\' LINES TERMINATED BY '\r\n' IGNORE 1 LINES;