select top 1 id from

返回查询结果中前几条数据

select count(1) as total 

返回查询的记录个数

select max(UserId) as total from 

返回查询内容的最大值或最大长度

select LEN(m.ModuleCode) AS [Length],'00000000000' AS cPower FROM Menu AS m ORDER BY CAST(m.ModuleCode AS INT)

CAST表示将里面的内容转化为对应的数据类型

JOIN 分为内连接，外连接（左外连接，右外连接，全外连接）

内连接： SELECT A.* FROM A INNER JOIN B ON A.ID=B.ID

SELECT a.cCardNo, a.cDoorId, a.cTimeId, b.vEmp_name, b.vDepart, b.dEndDate

FROM MJ_MacPower AS a INNER JOIN

Employee AS b ON a.cCardNo = b.vCardNo

左外连接： SELECT A.* FROM A LEFT JOIN B ON A.ID = B.ID

右外连接： SELECT A.* FROM A RIGHT JOIN B ON A.ID = B.ID

全外连接： SELECT A.*, B.* FROM A FULL OUTER搜索 JOIN B ON A.ID = B.ID