Review: Basic Knowledge about SQL

时间：2023-03-10 03:31:25
Review: Basic Knowledge about SQL

转自Github：enochtangg/quick-SQL-cheatsheet

目录

  1. 查找数据的查询
  2. 修改数据的查询
  3. 聚合查询
  4. 连接查询
  5. 视图查询
  6. 修改表的查询

1. 查找数据的查询

1.1 SELECT: 用于从数据库中选择数据

SELECT * FROM table_name;

1.2 DISTINCT: 用于过滤掉重复的值并返回指定列的行

 SELECT DISTINCT column_name;

1.3 WHERE: 用于过滤记录/行

 SELECT column1, column2 FROM table_name WHERE condition;

 SELECT * FROM table_name WHERE condition1 AND condition2;

 SELECT * FROM table_name WHERE condition1 OR condition2;

 SELECT * FROM table_name WHERE NOT condition;

 SELECT * FROM table_name WHERE condition1 AND (condition2 OR condition3);

 SELECT * FROM table_name WHERE EXISTS (SELECT column_name FROM table_name WHERE condition);

1.4 ORDER BY: 用于结果集的排序，升序（ASC）或者降序（DESC）

 SELECT * FROM table_name ORDER BY column;

 SELECT * FROM table_name ORDER BY column DESC;

 SELECT * FROM table_name ORDER BY column1 ASC, column2 DESC;

1.5 SELECT TOP: 用于指定从表顶部返回的记录数

 SELECT TOP number columns_names FROM table_name WHERE condition;

 SELECT TOP percent columns_names FROM table_name WHERE condition;

 --并非所有数据库系统都支持SELECT TOP。 MySQL 中是LIMIT子句

 SELECT column_names FROM table_name LIMIT offset, count;

1.6 LIKE: 用于搜索列中的特定模式，WHERE 子句中使用的运算符

% (percent sign) 是一个表示零个，一个或多个字符的通配符

_ (underscore) 是一个表示单个字符通配符

 SELECT column_names FROM table_nam

 WHERE column_name LIKE pattern;

pattern可以如下

 LIKE ‘a%’    （查找任何以“a”开头的值）

 LIKE ‘%a’    （查找任何以“a”结尾的值）

 LIKE ‘%or%’  （查找任何包含“or”的值）

 LIKE ‘_r%’   （查找任何第二位是“r”的值）

 LIKE ‘a_%_%’ （查找任何以“a”开头且长度至少为3的值）

 LIKE ‘[a-c]%’（查找任何以“a”或“b”或“c”开头的值）

1.7 IN: 用于在 WHERE 子句中指定多个值的运算符

本质上，IN运算符是多个OR条件的简写

 SELECT column_names FROM table_name

 WHERE column_name IN (value1, value2, …);

 SELECT column_names FROM table_name

 WHERE column_name IN (SELECT STATEMENT);

1.8 BETWEEN: 用于过滤给定范围的值的运算符

 SELECT column_names FROM table_name

 WHERE column_name BETWEEN value1 AND value2;

 SELECT * FROM Products

 WHERE (column_name BETWEEN value1 AND value2) AND NOT column_name2 IN (value3, value4);

 SELECT * FROM Products WHERE column_name BETWEEN #01/07/1999# AND #03/12/1999#;

1.9 NULL: 代表一个字段没有值

 SELECT * FROM table_name

 WHERE column_name IS NULL;

 SELECT * FROM table_name

 WHERE column_name IS NOT NULL;

1.10 AS: 用于给表或者列分配别名

 SELECT column_name AS alias_name FROM table_name;

 SELECT column_name FROM table_name AS alias_name;

 SELECT column_name AS alias_name1, column_name2 AS alias_name2;

 SELECT column_name1, column_name2 + ‘, ‘ + column_name3 AS alias_name;

1.11 UNION: 用于组合两个或者多个 SELECT 语句的结果集的运算符

  • 每个 SELECT 语句必须拥有相同的列数
  • 列必须拥有相似的数据类型
  • 每个 SELECT 语句中的列也必须具有相同的顺序
 SELECT columns_names FROM table1

 UNION SELECT column_name FROM table2;
  • UNION 仅允许选择不同的值, UNION ALL 允许重复

1.12 ANY|ALL: 用于检查 WHERE 或 HAVING 子句中使用的子查询条件的运算符

  • ANY 如果任何子查询值满足条件，则返回 true。
  • ALL 如果任何子查询值满足条件，则返回 true。
 SELECT columns_names FROM table1

 WHERE column_name operator (ANY|ALL) (SELECT column_name FROM table_name WHERE condition);

1.13 GROUP BY: 通常与聚合函数（COUNT，MAX，MIN，SUM，AVG）一起使用，用于将结果集分组为一列或多列

 SELECT column_name1, COUNT(column_name2) FROM table_name

 WHERE condition

 GROUP BY column_name1

 ORDER BY COUNT(column_name2) DESC;

1.14 HAVING: HAVING 子句指定 SELECT 语句应仅返回聚合值满足指定条件的行。它被添加到 SQL 语言中，因为WHERE关键字不能与聚合函数一起使用。

 SELECT COUNT(column_name1), column_name2 FROM table

  GROUP BY column_name2

  HAVING COUNT(column_name1) > 5;

2. 修改数据的查询

2.1 INSERT INTO: 用于在表中插入新记录/行

 INSERT INTO table_name (column1, column2) VALUES (value1, value2);

 INSERT INTO table_name VALUES (value1, value2 …);

2.2 UPDATE: 用于修改表中的现有记录/行

 UPDATE table_name

 SET column1 = value1, column2 = value2

 WHERE condition; 

 UPDATE table_name

 SET column_name = value;

2.3 DELETE: 用于删除表中的现有记录/行

 DELETE FROM table_name

 WHERE condition;

 DELETE * FROM table_name;

3. 聚合查询

3.1 COUNT: 返回出现次数

 SELECT COUNT (DISTINCT column_name);

3.2 MIN() and MAX(): 返回所选列的最小/最大值

 SELECT MIN (column_names) FROM table_name

 WHERE condition;

 SELECT MAX (column_names) FROM table_name

 WHERE condition;

3.3 AVG(): 返回数字列的平均值

SELECT AVG (column_name) FROM table_name

WHERE condition;

3.4 SUM(): 返回数值列的总和

 SELECT SUM (column_name) FROM table_name

 WHERE condition;

4. 连接查询

4.1 INNER JOIN: 内连接，返回在两张表中具有匹配值的记录

 SELECT column_names

 FROM table1 INNER JOIN table2

 ON table1.column_name=table2.column_name;

 SELECT table1.column_name1, table2.column_name2, table3.column_name3

 FROM

 (

    (table1 INNER JOIN table2 ON relationship)

    INNER JOIN

    table3

    ON relationship

 );

4.2 LEFT (OUTER) JOIN: 左外连接，返回左表（table1）中的所有记录，以及右表中的匹配记录（table2）

 SELECT column_names

 FROM table1 LEFT JOIN table2

 ON table1.column_name=table2.column_name;

4.3 RIGHT (OUTER) JOIN: 右外连接，返回右表（table2）中的所有记录，以及左表（table1）中匹配的记录

 SELECT column_names

 FROM table1 RIGHT JOIN table2

 ON table1.column_name=table2.column_name;

4.4 FULL (OUTER) JOIN: 全外连接，全连接是左右外连接的并集. 连接表包含被连接的表的所有记录, 如果缺少匹配的记录, 以 NULL 填充。

 SELECT column_names

 FROM table1 FULL OUTER JOIN table2

 ON table1.column_name=table2.column_name;

4.5 Self JOIN: 自连接，表自身连接

 SELECT column_names

 FROM table1 T1, table1 T2

 WHERE condition;

5. 视图查询

5.1 CREATE: 创建视图

 CREATE VIEW view_name

 AS

 SELECT column1, column2

 FROM table_name WHERE condition;

5.2 SELECT: 检索视图

 SELECT * FROM view_name;

5.3 DROP: 删除视图

 DROP VIEW view_name;

6. 修改表的查询

6.1 ADD: 添加字段

 ALTER TABLE table_name ADD column_name column_definition;

6.2 MODIFY: 修改字段数据类型

 ALTER TABLE table_name MODIFY column_name column_type;

6.3 DROP: 删除字段

 ALTER TABLE table_name DROP COLUMN column_name;