mysql - 在 MySQL 中获取未保留上个月的用户

Question

我正在使用MySQL数据库，我有两个表。他们是User并且Reservation

这是我的问题。

1. 目前，我将LEFT JOINSubQuery 与NOT EXIST. 从性能的角度来看，哪个更好？
2. 我可以为此查询创建视图吗，这会对性能产生任何影响吗

用户

| FIELD |        TYPE | NULL | KEY | DEFAULT |          EXTRA |
|-------|-------------|------|-----|---------|----------------|
|   uid |     int(11) |   NO | PRI |  (null) | auto_increment |
| uname | varchar(30) |  YES |     |  (null) |                |

预订

|    FIELD |      TYPE | NULL | KEY |           DEFAULT |          EXTRA |
|----------|-----------|------|-----|-------------------|----------------|
|      rid |   int(11) |   NO | PRI |            (null) | auto_increment |
|      uid |   int(11) |  YES | MUL |            (null) |                |
| reserved | timestamp |   NO |     | CURRENT_TIMESTAMP |                |

SQL 代码：

create table user (
 uid int not null auto_increment,
 uname varchar(30),
 primary key(uid)
);

create table reservation (
 rid int not null auto_increment,
 uid int,
 reserved timestamp not null default CURRENT_TIMESTAMP,
 primary key(rid),
 foreign key (uid) references user (uid)
)

我当前工作的 SQL 查询

SELECT u.uid, u.uname, date_format(reserved, '%Y%m')  
FROM user as u 
LEFT JOIN reservation as r on  
r.uid = u.uid and date_format(reserved, '%Y%m') = 201307  
where r.uid is null  

Answer 1

Here's an excellent article about performance differences: NOT IN vs. NOT EXISTS vs. LEFT JOIN / IS NULL: MySQL

Summary:

...the best way to search for missing values in MySQL is using a LEFT JOIN / IS NULL or NOT IN rather than NOT EXISTS.

But you can improve performance a little by putting an index on reserved column and rewriting your query like this:

reserved >= '2013-07-01 00:00:00' AND reserved < '2013-08-01 00:00:00'

Views don't change anything of the performance of the query.

Answer 2

问题是，如果你做了一个date_formatMySQL 索引是没有用的。你应该使用这样的东西：

reserved >= '2013-07-01 00:00:00' AND reserved < '2013-08-01 00:00:00'

比使用索引，您的查询会更快。如果您在表预订上有一个组合索引，带有字段，uid, reserved

Answer 3

如果不存在，这会更好。

SELECT u.uid, u.uname, date_format(reserved, '%Y%m')  
FROM user as u where not exist (select 1 from reservation as r   
where r.uid = u.uid and date_format(reserved, '%Y%m') = 201307)

视图对性能没有多大帮助。它主要关注可重用性、数据隐藏和安全原因。

Answer 4

You'll want to use NOT EXISTS, and making a view won't really do much for you, even convenience wise since this is a pretty simple query.

Also see: SQL performance on LEFT OUTER JOIN vs NOT EXISTS