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SQL 速查
2020-11-08 16:57:00 【程序猿欧文】
- 关系数据库,基于关系模型,使用关系(表)存储数据,同时定义了完整性约束。常见的关系数据库系统包括:Oracle、MySQL/MariaDB、SQL Server、PostgreSQL 等等。
- SQL,结构化查询语言,访问和操作关系数据库的标准语言。SQL 具有声明性,是一种面向集合的编程语言。
1、单表查询
SELECT col1, col2 AS c2 -- 列别名 FROM t; -- 基本查询SELECT * FROM t; -- 查询所有字段SELECT col1, col2, … FROM t WHERE conditions; -- 过滤条件SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC; -- 排序SELECT col1, col2, … FROM t ORDER BY col1 ASC, col2 DESC OFFSET m ROWS FETCH FIRST n ROWS ONLY; -- 限定数量 LIMIT n OFFSET m; -- 非标准实现SELECT col1, col2, agg_fun() -- 聚合函数 FROM t GROUP BY col1, col2 -- 分组汇总 HAVING conditions; -- 分组后过滤
2、多表连接
SELECT t1.col1, t2.col2, … FROM t1 INNER JOIN t2 ON conditions; -- 内连接 SELECT t1.col1, t2.col2, … FROM t1 LEFT JOIN t2 ON conditions; -- 左连接SELECT t1.col1, t2.col2, … FROM t1 RIGHT JOIN t2 ON conditions; -- 右连接SELECT t1.col1, t2.col2, … FROM t1 FULL JOIN t2 ON conditions; -- 全连接SELECT t1.col1, t2.col2, … FROM t1 CROSS JOIN t2 ON conditions;-- 交叉连接SELECT a.col1, b.col2, … FROM t1 a -- 表别名 JOIN t1 b ON conditions; -- 自连接
3、集合运算
SELECT col1, col2, … FROM t1 UNION [ALL]SELECT c1, c2, … FROM t2; -.........
版权声明
本文为[程序猿欧文]所创,转载请带上原文链接,感谢
https://my.oschina.net/mikeowen/blog/4708273
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