1. 聚合函数

1.1

聚合函数 ：计算从列中取得的值，返回一个值
- AVG() - 平均数
- COUNT() - 计数
  - COUNT(column_name)：指定列的值的数目（NULL不计入）
  - COUNT(*)：表中的记录数
  - COUNT(DISTINCT column_name)：指定列的不同值的数目
- FIRST(column_name) - 指定列第一个记录的值（只有MS Access支持）
  - SQL Server语法：SELECT TOP 1 column_name FROM table_name;
  - MySQL语法：SELECT column_name FROM table_name LIMIT 1;
  - Oracle语法：SELECT column_name FROM table_name WHERE ROWNUM <=1;
- LAST(column_name) - 指定列最后一个记录的值（只有MS Access支持）
  - SQL Server语法：SELECT TOP 1 column_name FROM table_name ORDER BY id DESC;（id为自增ID）
  - MySQL语法：SELECT column_name FROM table_name ORDER BY id DESC LIMIT 1;
  - Oracle语法：SELECT column_name FROM table_name ORDER BY id DESC WHERE ROWNUM <=1;
- MAX() - 最大值
- MIN() - 最小值
- SUM() - 和

1.2

GROUP BY语句结合聚合函数，根据一个或多个列对多个结果集进行分组。

 SELECT column_name, aggregate_function(column_name)
 FROM table_name
 WHERE column_name operator value
 GROUP BY column_name;

GROUP BY 多表连接

SELECT table_1.column_name，aggregate_function(table_2.column_name) AS column_new_name 
FROM table_2 LEFT JOIN table_1 
ON table_2.id = table_id
GROUP BY table_1.column_name;

HAVING字句筛选分组后的各组数据（WHERE关键字无法与聚合函数一起使用）

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value;

2. 纯量函数

纯量函数 ：基于输入值，返回一个值
- UCASE() - 大写
  - SQL语法：SELECT UCASE(column_name) FROM table_name;
  - SQL Server语法：SELECT UPPER(column_name) FROM table_name;
- LCASE() - 小写
  - SQL语法：SELECT LCASE(column_name) FROM table_name;
  - SQL Server语法：SELECT LOWER(column_name) FROM table_name;
- MID() - 从某个文本字段提取字符
  - SELECT MID(column_name, start, length) FROM table_name。start 必需，规定开始位置，起始值是1；length可选，省略的话返回剩余文本。
- SubString(字段, 1, end) - 从某个文本字段提取字符
- LEN() - 长度
  - SQL语法：SELECT LEN(column_name) FROM table_name;
  - MySQL语法：SELECT LENGTH(column_name) FROM table_name;
- ROUND(column_name, decimals) - 把数值字段舍入为指定的小数位数。
  - decimals规定要返回的小数位数，如果decimals为0，结果将没有小数部分。
- NOW() - 当前系统日期和时间
  - SELECT NOW() FROM table_name;
- FORMAT() - 显示格式化
  - SELECT FORMAT(column_name, format) FROM table_name;format为规定格式，如 DATE_FORMAT(NOW(), ‘%Y-%m-%。

3. SQL快速参考

AND / OR - 条件与或

SELECT column_name(s)
FROM table_name
WHERE condition
AND|OR condition

ALTER TABLE - 修改表

ALTER TABLE table_name 
ADD column_name datatype
或
ALTER TABLE table_name 
DROP COLUMN column_name

AS (alias) - 别名

SELECT column_name AS column_alias
FROM table_name
或
SELECT column_name
FROM table_name AS table_alias

BETWEEN - 之间

SELECT column_name(s)
FROM table_name
WHERE column_name
BETWEEN value1 AND value2

CREATE DATABASE - 创建数据库

CREATE DATABASE database_name

CREATE TABLE - 创建数据表

CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name2 data_type,
...
)

CREATE INDEX - 创建索引

CREATE INDEX index_name
ON table_name (column_name)
或
CREATE UNIQUE INDEX index_name
ON table_name (column_name)

CREATE VIEW - 创建视图

CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition

DELETE - 删除

DELETE FROM table_name
WHERE some_column=some_value
或
DELETE FROM table_name 
(Note: Deletes the entire table!!)
DELETE * FROM table_name 
(Note: Deletes the entire table!!)

DROP DATABASE - 删除数据库

DROP DATABASE database_name

DROP INDEX - 删除索引

DROP INDEX table_name.index_name (SQL Server)
DROP INDEX index_name ON table_name (MS Access)
DROP INDEX index_name (DB2/Oracle)
ALTER TABLE table_name
DROP INDEX index_name (MySQL)

DROP TABLE - 删除整个表

DROP TABLE table_name

GROUP BY - 分组

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name

HAVING - 选组

SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value

IN - 符合哪些值

SELECT column_name(s)
FROM table_name
WHERE column_name
IN (value1,value2,..)

INSERT INTO - 插入新记录

INSERT INTO table_name
VALUES (value1, value2, value3,....)
或
INSERT INTO table_name
(column1, column2, column3,...)
VALUES (value1, value2, value3,....)

INNER JOIN - 内连接

SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

LEFT JOIN - 左连接

SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

RIGHT JOIN - 右连接

SELECT column_name(s)
FROM table_name1
RIGHT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

FULL JOIN - 全连接

SELECT column_name(s)
FROM table_name1
FULL JOIN table_name2 
ON table_name1.column_name=table_name2.column_name

LIKE - 像某种格式

SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern

ORDER BY - 排序

SELECT column_name(s)
FROM table_name
ORDER BY column_name [ASC|DESC]

SELECT - 查询某（些）列

SELECT column_name(s)
FROM table_name

SELECT * - 查询所有列

SELECT *
FROM table_name

SELECT DISTINCT - 查询不重复的列

SELECT DISTINCT column_name(s)
FROM table_name

SELECT INTO - 查询并插入

SELECT *
INTO new_table_name [IN externaldatabase]
FROM old_table_name
或
SELECT column_name(s)
INTO new_table_name [IN externaldatabase]
FROM old_table_name

SELECT TOP - 查询前面的列值

SELECT TOP number|percent column_name(s)
FROM table_name

TRUNCATE TABLE - 删除表内容，保留表结构

TRUNCATE TABLE table_name

UNION - 联合

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

UNION ALL - 允许重复的值

SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2

UPDATE - 更改表

UPDATE table_name
SET column1=value, column2=value,...
WHERE some_column=some_value

WHERE - 条件

SELECT column_name(s)
FROM table_name
WHERE column_name operator value

4. 总结

SQL是用来访问和处理数据库系统的标准计算机语言，我们可以使用它在数据库中执行查询、获取数据、插入记录、删除记录、更新记录，还可以创建库、表、索引以及撤销它们。我们还应该掌握SQL中最重要的Aggregate函数。SQL是一种与数据库系统协同工作的标准语言，这些数据库包括MS SQL Server、IBM DB2、Oracle、MySQL和MS Access。

SQL 函数和SQL语句参考

1. 聚合函数

1.1

1.2

2. 纯量函数

3. SQL快速参考

4. 总结

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