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SQL 教程

SQL 首页 SQL 简介 SQL 语法 SQL 选择 SQL 选择不同 SQL 条件 SQL 排序 SQL 并且 SQL 或者 SQL 非 SQL 插入 SQL 空值 SQL 更新 SQL 删除 SQL 选择前几条 SQL 聚合函数 SQL 最小值和最大值 SQL 计数 SQL 求和 SQL 平均值 SQL 相似 SQL 通配符 SQL 在 SQL 之间 SQL 别名 SQL 连接 SQL 内连接 SQL 左连接 SQL 右连接 SQL 全连接 SQL 自连接 SQL 并集 SQL 分组 SQL 筛选分组 SQL 存在 SQL 任何, 所有 SQL 选择到 SQL 插入选择 SQL 条件语句 SQL 空值函数 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 创建表格 SQL 删除表格 SQL 修改表格 SQL 约束 SQL 非空 SQL 唯一 SQL 主键 SQL 外键 SQL 检查 SQL 默认值 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 + 连接字符串 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 例子

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

SQL CREATE UNIQUE INDEX 关键字

❮ 上一步 ❮ SQL 关键字 参考 下一步 ❯

CREATE UNIQUE INDEX

The CREATE UNIQUE INDEX 命令在表上创建唯一索引（不允许重复值）

索引用于非常快地从数据库中检索数据。用户无法看到索引，它们只用于加速搜索/查询。

以下 SQL 在“Persons” 表的“PersonID” 列上创建名为“uidx_pid” 的索引

CREATE UNIQUE INDEX uidx_pid
ON Persons (PersonID);

注意: 创建索引的语法在不同的数据库之间有所不同。因此：检查您的数据库中创建索引的语法。

注意: 更新带有索引的表比更新没有索引的表需要更多时间（因为索引也需要更新）。因此，只对将频繁搜索的列创建索引。


❮ 上一步 ❮ SQL 关键字 参考 下一步 ❯
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