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SQL函数（三）

分类: 文章 • 2025-01-25 21:06:22

 

Sql函数

 

 

 

 

Avg

计算id的平均值

SELECT AVG(id) FROM w_wkn

 

 

COUNT()

返回匹配指定条件的行数。

ELECT COUNT(id) FROM w_wkn

 

 

 FIRST()

函数返回指定的字段中第一个记录的值。

SELECT FIRST(OrderPrice) AS FirstOrderPrice FROM Orders

 

MAX() 函数

 函数返回一列中的最大值。NULL 值不包括在计算中。

SELECT MAX(id) FROM w_wkn

 

MIN() 函数

MIN 函数返回一列中的最小值。NULL 值不包括在计算中。

 

SUM() 函数

 函数返回数值列的总数（总额）。

 

 

GROUP BY 语句

GROUP BY 语句用于结合合计函数，根据一个或多个列对结果集进行分组。

 

根据name查询每个name的id的总合

SELECT SUM(id) FROM w_wkn GROUP BY w_name

 

 

HAVING 子句

在 SQL 中增加 HAVING 子句原因是，WHERE 关键字无法与合计函数一起使用。

 

SELECT w_name , SUM(id) FROM w_wkn GROUP BY w_address HAVING SUM(id) > 1

 

 

 

UCASE() 函数

UCASE 函数把字段的值转换为大写。

SELECT UCASE(w_name) FROM w_wkn

 

 

 

LCASE() 函数

LCASE 函数把字段的值转换为小写。

SELECT LCASE(w_name) FROM w_wkn

 

 

 

 

MID() 函数

MID 函数用于从文本字段中提取字符。

SELECT MID(w_name,1,2) FROM w_wkn

 

 

 

 

LEN() 函数

LEN 函数返回文本字段中值的长度。

 

 

 

 

ROUND() 函数

ROUND 函数用于把数值字段舍入为指定的小数位数。

 

 

NOW() 函数

NOW 函数返回当前的日期和时间。

 

 

 

 

FORMAT() 函数

FORMAT 函数用于对字段的显示进行格式化。

 

 

 

 

 

 

SQL 语句

语句

语法

AND / OR

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

ALTER TABLE (add column)

ALTER TABLE table_name 
ADD column_name datatype

ALTER TABLE (drop column)

ALTER TABLE table_name 
DROP COLUMN column_name

AS (alias for column)

SELECT column_name AS column_alias
FROM table_name

AS (alias for table)

SELECT column_name
FROM table_name  AS table_alias

BETWEEN

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

CREATE DATABASE

CREATE DATABASE database_name

CREATE INDEX

CREATE INDEX index_name
ON table_name (column_name)

CREATE TABLE

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

CREATE UNIQUE INDEX

CREATE UNIQUE INDEX index_name
ON table_name (column_name)

CREATE VIEW

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

DELETE FROM

DELETE FROM table_name 
(Note: Deletes the entire table!!)

or

DELETE FROM table_name
WHERE condition

DROP DATABASE

DROP DATABASE database_name

DROP INDEX

DROP INDEX table_name.index_name

DROP TABLE

DROP TABLE table_name

GROUP BY

SELECT column_name1,SUM(column_name2)
FROM table_name
GROUP BY column_name1

HAVING

SELECT column_name1,SUM(column_name2)
FROM table_name
GROUP BY column_name1
HAVING SUM(column_name2) condition value

IN

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

INSERT INTO

INSERT INTO table_name
VALUES (value1, value2,....)

or

INSERT INTO table_name
(column_name1, column_name2,...)
VALUES (value1, value2,....)

LIKE

SELECT column_name(s)
FROM table_name
WHERE column_name
LIKE pattern

ORDER BY

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

SELECT

SELECT column_name(s)
FROM table_name

SELECT *

SELECT *
FROM table_name

SELECT DISTINCT

SELECT DISTINCT column_name(s)
FROM table_name

SELECT INTO
(used to create backup copies of tables)

SELECT *
INTO new_table_name
FROM original_table_name

or

SELECT column_name(s)
INTO new_table_name
FROM original_table_name

TRUNCATE TABLE
(deletes only the data inside the table)

TRUNCATE TABLE table_name

UPDATE

UPDATE table_name
SET column_name=new_value
[, column_name=new_value]
WHERE column_name=some_value

WHERE

SELECT column_name(s)
FROM table_name
WHERE condition
UNION的用法

SQL函数（三）




SELECT w_wkn.w_name from w_wkn 
WHERE id>1 
UNION 
SELECT w_wkn.w_address FROM w_wkn WHERE w_name LIKE 'a'


SQL函数（三）













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