MySQL 引号

MySQL里引用字符串常量时需要用一堆英文单引号 ' 或英文双引号 " 讲字符串常量括起来

数据库查询

SELECT * FROM user WHERE DATABASE()="practice" LIMIT 1 OFFSET 2;

SELECT * FROM user WHERE DATABASE()="practice" LIMIT 2, 1;

LIMIT

LIMIT X, Y中X表示跳过X个数据，读取Y个数据
LIMIT Y表示选取Y个数据

OFFSET

OFFSET X表示跳过X个数据

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WHERE 字句

SELECT * FROM user WHERE username="hello" AND password <> "world";

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UPDATE 更新

UPDATE user SET username="hello", password="world!" WHERE DATABASE()="practice" AND username="hello" AND password="world";

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DELETE 语句

DELETE FROM user WHERE username="" AND password="";

DELETE、TRUNCATE和DROP的区别

DROP > TRUNCATE > DELETE

DELETE

• delete是DML，执行delete操作时，每次从表中删除一行，并且同时将该行的删除操作记录在redo和undo表空间中以便进行回滚(rollback)和重做操作，但要注意表空间要足够大，需要手动提交才能生效
• delete可根据条件删除表中满足条件的数据，如果不指定where字句，那么删除表中所有记录
• delete语句不影响表所占用的extent，高水线(high watermark)保持原位置不变

TRUNCATE

• TRUNCATE是DDL，会隐式提交，所以不会回滚，不会触发触发器
• 会删除表中所有记录，并且重新设置高水线和所有的索引
• 对于外键约束引用的表，不能用truncate table，而应使用不带WHERE字句的DELETE语句
• 不能用于参与了索引视图的表

DROP

• DROP是DDL，会隐式提交，所以，不能回滚，不会触发触发器。
• drop语句删除表结构及所有数据，并将表所占用的空间全部释放。
• drop语句将删除表的结构所依赖的约束，触发器，索引，依赖于该表的存储过程/函数将保留，但是变为invalid状态

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LIKE 语句

SELECT * FROM user WHERE password LIKE "%2%";

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UNION 语句

SELECT country FROM apps WHERE id=1 UNION DISTINCT SELECT country FROM websites WHERE id=1;

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MySQL 排序

SELECT * FROM user ORDER BY password ASC;

SELECT * FROM user ORDER BY password DESC;

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MySQL 分组

SELECT name, COUNT(name) from employee_tbl GROUP BY name, signin;

Coalesce(a, b, c)

SELECT COALESCE(name, "总数") as "姓名", COUNT(signin) FROM employee_tbl GROUP BY name WITH ROLLUP;

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MySQL 连接的使用

INNER JOIN

内连接，或等值连接。
获取两个表中字段匹配关系的记录。

SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl AS a INNER JOIN tcount_tbl AS b ON a.runoob_author = b.runoob_author;

LEFT JOIN

左连接
获取左表所有记录，即使右表没有对应匹配的记录。

SELECT a.runoob_id, a.runoob_title, b.runoob_count, b.runoob_author FROM RUNOOB_TBL AS a LEFT JOIN tcount_tbl AS b ON a.runoob_author = b.runoob_author;

RIGHT JOIN

右连接
与左连接相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

SELECT a.runoob_id, a.runoob_title, b.runoob_author, b.runoob_count FROM runoob_tbl AS a RIGHT JOIN tcount_tbl AS b ON a.runoob_author = b.runoob_author;

标签：runoob,author,user,MySQL,WHERE,SELECT,tbl
来源： https://www.cnblogs.com/amazingman113/p/16086249.html