MySQL实践笔记

关于外键

子表：

CREATE TABLE req(
  icon VARCHAR(128) PRIMARY KEY,
  series VARCHAR(128),
  CONSTRAINT fk_series FOREIGN KEY(series) REFERENCES series(name) ON UPDATE CASCADE ON DELETE SET NULL
) ENGINE = InnoDB;

父表：

CREATE TABLE series(
  name VARCHAR(128) PRIMARY KEY,
  sys TINYINT(1) DEFAULT 0
) ENGINE = InnoDB;

外键

支持外键只能使用 InnoDB 存储引擎。

级联操作问题

ON UPDATE CASCADE 指定级联更新；ON DELETE SET NULL 指定父表删除记录时子表相应字段设为 NULL。如果级联删除则是 ON DELETE CASCADE。

如果不使用这些语句，那么在父表更新或删除有子表依赖的记录时将不被允许。

为已有表增加外键

ALTER TABLE req ADD series VARCHAR(128);

ALTER TABLE req ADD CONSTRAINT fk_series FOREIGN KEY(series) REFERENCES series(name) ON UPDATE CASCADE ON DELETE SET NULL;

关于 SELECT 合并

SQL：

SELECT COUNT(*) AS sum FROM t1 WHERE p1 = 'x';
SELECT COUNT(*) AS sum FROM t1 WHERE p1 = 'x' AND p2 = 'y';

结果：

+-----+
| sum |
+-----+
|   4 |
+-----+

+-----+
| sum |
+-----+
|   1 |
+-----+

合并SQL：

SELECT COUNT(*) FROM t1 WHERE p1 = 'x' GROUP BY p2 = 'y';

结果：

+-----+
| sum |
+-----+
|   3 |
|   1 |
+-----+

通过 GROUP BY p2 = 'y' 将结果分成了两组

了解表结构

desc t1;

show create table t1;

GROUP BY 后查总数

select count(*) from (select pkg from req group by pkg);

这样是不行的。[Err] 1248 - Every derived table must have its own alias
正确语句如下：

select count(*) from (select pkg from req group by pkg) as t;

连接数

SHOW VARIABLES LIKE '%max_connections%';
SHOW GLOBAL STATUS LIKE '%max_used_connections%';
SET GLOBAL max_connections = 256;
SHOW PROCESSLIST;

关于 TIMESTAMP

time TIMESTAMP NOT NULL

等同于

time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

或许我们的需求是这样：

time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP