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SQL 教程

SQL 主页 SQL 简介 SQL 语法 SQL Select SQL Select Distinct SQL Where SQL Order By SQL And SQL Or SQL Not SQL Insert Into SQL Null Values SQL Update SQL Delete SQL Select Top SQL 聚合函数 SQL Min 和 Max SQL Count SQL Sum SQL Avg SQL Like SQL 通配符 SQL In SQL Between SQL 别名 SQL 联接 SQL Inner Join SQL Left Join SQL Right Join SQL Full Join SQL 自联接 SQL Union SQL Group By SQL Having SQL Exists SQL Any, All SQL Select Into SQL Insert Into Select SQL Case SQL Null 函数 SQL 存储过程 SQL 注释 SQL 运算符

SQL 数据库

SQL 创建数据库 SQL 删除数据库 SQL 备份数据库 SQL 创建表 SQL 删除表 SQL 修改表 SQL 约束 SQL Not Null SQL Unique SQL 主键 SQL 外键 SQL Check SQL Default SQL 索引 SQL 自增 SQL 日期 SQL 视图 SQL 注入 SQL 托管 SQL 数据类型

SQL 参考

SQL 关键字
ADD ADD CONSTRAINT ALL ALTER ALTER COLUMN ALTER TABLE AND ANY AS ASC BACKUP DATABASE BETWEEN CASE CHECK COLUMN CONSTRAINT CREATE CREATE DATABASE CREATE INDEX CREATE OR REPLACE VIEW CREATE TABLE CREATE PROCEDURE CREATE UNIQUE INDEX CREATE VIEW DATABASE DEFAULT DELETE DESC DISTINCT DROP DROP COLUMN DROP CONSTRAINT DROP DATABASE DROP DEFAULT DROP INDEX DROP TABLE DROP VIEW EXEC EXISTS FOREIGN KEY FROM FULL OUTER JOIN GROUP BY HAVING IN INDEX INNER JOIN INSERT INTO INSERT INTO SELECT IS NULL IS NOT NULL JOIN LEFT JOIN LIKE LIMIT NOT NOT NULL OR ORDER BY OUTER JOIN PRIMARY KEY PROCEDURE RIGHT JOIN ROWNUM SELECT SELECT DISTINCT SELECT INTO SELECT TOP SET TABLE TOP TRUNCATE TABLE UNION UNION ALL UNIQUE UPDATE VALUES VIEW WHERE
MySQL 函数
字符串函数: ASCII CHAR_LENGTH CHARACTER_LENGTH CONCAT CONCAT_WS FIELD FIND_IN_SET FORMAT INSERT INSTR LCASE LEFT LENGTH LOCATE LOWER LPAD LTRIM MID POSITION REPEAT REPLACE REVERSE RIGHT RPAD RTRIM SPACE STRCMP SUBSTR SUBSTRING SUBSTRING_INDEX TRIM UCASE UPPER 数值函数: ABS ACOS ASIN ATAN ATAN2 AVG CEIL CEILING COS COT COUNT DEGREES DIV EXP FLOOR GREATEST LEAST LN LOG LOG10 LOG2 MAX MIN MOD PI POW POWER RADIANS RAND ROUND SIGN SIN SQRT SUM TAN TRUNCATE 日期函数: ADDDATE ADDTIME CURDATE CURRENT_DATE CURRENT_TIME CURRENT_TIMESTAMP CURTIME DATE DATEDIFF DATE_ADD DATE_FORMAT DATE_SUB DAY DAYNAME DAYOFMONTH DAYOFWEEK DAYOFYEAR EXTRACT FROM_DAYS HOUR LAST_DAY LOCALTIME LOCALTIMESTAMP MAKEDATE MAKETIME MICROSECOND MINUTE MONTH MONTHNAME NOW PERIOD_ADD PERIOD_DIFF QUARTER SECOND SEC_TO_TIME STR_TO_DATE SUBDATE SUBTIME SYSDATE TIME TIME_FORMAT TIME_TO_SEC TIMEDIFF TIMESTAMP TO_DAYS WEEK WEEKDAY WEEKOFYEAR YEAR YEARWEEK 高级函数: BIN BINARY CASE CAST COALESCE CONNECTION_ID CONV CONVERT CURRENT_USER DATABASE IF IFNULL ISNULL LAST_INSERT_ID NULLIF SESSION_USER SYSTEM_USER USER VERSION
SQL Server 函数
字符串函数: ASCII CHAR CHARINDEX CONCAT 使用 + 连接 CONCAT_WS DATALENGTH DIFFERENCE FORMAT LEFT LEN LOWER LTRIM NCHAR PATINDEX QUOTENAME REPLACE REPLICATE REVERSE RIGHT RTRIM SOUNDEX SPACE STR STUFF SUBSTRING TRANSLATE TRIM UNICODE UPPER 数值函数: ABS ACOS ASIN ATAN ATN2 AVG CEILING COUNT COS COT DEGREES EXP FLOOR LOG LOG10 MAX MIN PI POWER RADIANS RAND ROUND SIGN SIN SQRT SQUARE SUM TAN 日期函数: CURRENT_TIMESTAMP DATEADD DATEDIFF DATEFROMPARTS DATENAME DATEPART DAY GETDATE GETUTCDATE ISDATE MONTH SYSDATETIME YEAR 高级函数 CAST COALESCE CONVERT CURRENT_USER IIF ISNULL ISNUMERIC NULLIF SESSION_USER SESSIONPROPERTY SYSTEM_USER USER_NAME
MS Access 函数
字符串函数: Asc Chr 使用 & 连接 CurDir Format InStr InstrRev LCase Left Len LTrim Mid Replace Right RTrim Space Split Str StrComp StrConv StrReverse Trim UCase 数值函数: Abs Atn Avg Cos Count Exp Fix Format Int Max Min Randomize Rnd Round Sgn Sqr Sum Val 日期函数: Date DateAdd DateDiff DatePart DateSerial DateValue Day Format Hour Minute Month MonthName Now Second Time TimeSerial TimeValue Weekday WeekdayName Year 其他函数: CurrentUser Environ IsDate IsNull IsNumeric
SQL 快速参考

SQL 示例

SQL 示例 SQL 编辑器 SQL 测验 SQL 练习 SQL Server SQL 集训营 SQL 证书

SQL Server LEFT() 函数

❮ 上一个 ❮ SQL Server 函数 下一个 ❯

示例

从字符串中提取 3 个字符（从左侧开始）

SELECT LEFT('SQL Tutorial', 3) AS ExtractString;
亲自尝试 »

定义和用法

LEFT() 函数从字符串中提取一定数量的字符（从左侧开始）。

语法

LEFT(string, number_of_chars)

参数值

参数 描述
string 必需。要提取的字符串
number_of_chars 必需。要提取的字符数。如果数字超过string 中的字符数，则返回string

技术细节

适用范围 SQL Server（从 2008 开始）、Azure SQL 数据库、Azure SQL 数据仓库、并行数据仓库

更多示例

示例

从 "CustomerName" 列中的文本中提取 5 个字符（从左侧开始）

SELECT LEFT(CustomerName, 5) AS ExtractString
FROM Customers;
亲自尝试 »

示例

从字符串中提取 100 个字符（从左侧开始）

SELECT LEFT('SQL Tutorial', 100) AS ExtractString;
亲自尝试 »

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