连接查询

连接查询：列的扩展
1. 等值连接查询 (内连接)
笛卡尔积
join on

2. 外连接（left.right.full）

left左连接：
left 左边表为主表 ，右边的为从表，显示的结果集以主表为主
如果从表中没有和它匹配的数据，就以NULL值填充

right右连接：
right 右边的表为主表

full 全连接
每个表都完全显示，互相匹配数据，如果对方没有则以NULL填充

联合查询：行的扩展

用Union将两个查询联合起来显示，追加行的形式在结果集中显示
必须保证两个查询结果的列的格式是一样的

子查询

1.无关子查询：查询嵌套，外层查询成为主查询，内层查询成为子查询
子查询的结果成为主查询的条件

select Info.Code,Info.Name,Nation.Name from Info,Nation where Nation.Code = Info.Nation

SELECT Info.Code,Info.Name,Nation.Name,Family.Name from Info JOIN Nation on Nation.Code = Info.Nation JOIN Family on Info.Code=Family.InfoCode

SELECT Info.Code,Info.Name,Nation.Name from Info right JOIN Nation on Nation.Code = Info.Nation

SELECT Info.Code,Info.Name,Nation.Name from Info full JOIN Nation on Nation.Code = Info.Nation

SELECT Code,Name from Info
UNION
SELECT code,Name from Nation

select * FROM Info where YEAR(Birthday) = (SELECT YEAR(Birthday) from Info where Code = ‘p005‘)

SELECT TOP 5 * from News where ids NOT IN (SELECT TOP 10 ids FROM News)

SELECT * from score where degree>ALL(SELECT degree from score where sno=‘105‘)