SQL Server T-SQL高级查询、函数

高级查询在数据库中用得是最频繁的，也是应用最广泛、最普遍的。

Ø 基本常用查询

-- all 查询所有,几乎从来不用 all 关键字,因为是默认关键字

select all sex from student;

-- distinct 过滤重复 (常用语检查一列数据是否有异常值)

select distinct sex from student;

-- count 统计

select count(distinct sex) from student;

-- top 取前N条记录

select top 3 * from student;

-- column 列运算

select (age + id) col from student;

select s.name + '-' + c.name from classes c, student s where s.cid = c.id;

-- having 分组过滤条件

-- 按照年龄分组，过滤年龄为空的数据，并且统计分组的条数和现实年龄信息

select count(*), age from student group by age having age is not null;

-- 按照年龄和cid组合分组，过滤条件是cid大于1的记录

select count(*), cid, sex from student group by cid, sex having cid > 1;

-- 按照年龄分组，过滤条件是分组后的记录条数大于等于2

select count(*), age from student group by age having count(age) >= 2;

-- 按照cid和性别组合分组，过滤条件是cid大于1，cid的最大值大于2

select count(*), cid, sex from student group by cid, sex having cid > 1 and max(cid) > 2;