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SQL 教程

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SQL 参考资料

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 + 拼接字符串 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 & 拼接字符串 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

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SQL COUNT() 函数

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SQL COUNT() 函数

The COUNT() function returns the number of rows that matches a specified criterion.

示例

在 Products 表中查找总行数

SELECT COUNT(*)
FROM Products;
亲自尝试 »

语法

SELECT COUNT(column_name)
FROM table_name
WHERE condition;

演示数据库

以下是示例中使用的 Products 表的选定部分

ProductID ProductName SupplierID CategoryID Unit Price
1 Chais 1 1 10 boxes x 20 bags 18
2 Chang 1 1 24 - 12 oz bottles 19
3 Aniseed Syrup 1 2 12 - 550 ml bottles 10
4 Chef Anton's Cajun Seasoning 2 2 48 - 6 oz jars 22
5 Chef Anton's Gumbo Mix 2 2 36 boxes 21.35

指定列

您可以指定列名而不是星号 (*)。

如果您指定列名而不是 (*)，则不会计算 NULL 值。

示例

查找 ProductName 不为 NULL 的产品数量

SELECT COUNT(ProductName)
FROM Products;
亲自尝试 »

添加 WHERE 子句

您可以添加 WHERE 子句来指定条件

示例

查找 Price 大于 20 的产品数量

SELECT COUNT(ProductID)
FROM Products
WHERE Price > 20;
亲自尝试 »

忽略重复项

您可以使用 COUNT() 函数中的 DISTINCT 关键字来忽略重复项。

如果指定了 DISTINCT，则具有相同列值的记录将被计为一个。

示例

在 Products 表中，有多少种不同的价格？

SELECT COUNT(DISTINCT Price)
FROM Products;
亲自尝试 »

使用别名

使用 AS 关键字为计数列命名。

示例

将列命名为 "Number of records"

SELECT COUNT(*) AS [Number of records]
FROM Products;
亲自尝试 »

将 COUNT() 与 GROUP BY 一起使用

在这里，我们使用 COUNT() 函数和 GROUP BY 子句，返回 Products 表中每个类别中的记录数

示例

SELECT COUNT(*) AS [Number of records], CategoryID
FROM Products
GROUP BY CategoryID;
亲自尝试 »

您将在本教程的后面部分详细了解 GROUP BY 子句。

通过练习测试自己

练习

使用正确的函数返回具有 Price 值设置为 18 的记录数。

SELECT (*)
FROM Products
 Price = 18;

开始练习


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