一.聯合查詢：將多次查詢(多條select語句)，在記錄上拼接(字段不會增加)

????????? 1. 基本語法：多條select構成，每條select語句字段數保持一致(字段類型可以不一致)

??????????????????????????????? 1)：select?? 語句1 union[選項] select 語句2 union[選項]

??????????????????????????????? 2)：union[選項]：all(不管重復)保留所有、distinct去重(必須一條記錄全部重復）默認

?????????????????????????????????????? select * from my_class union distinct select name,sex,course from student;

??????????????????????????????? 3)：聯合查詢的意義

?????????????????????????????????????????????? 1.查詢同一張表，但需求不同，男生降序，女生升序

?????????????????????????????????????????????????? 在聯合查詢中使用order by查詢兩個查詢語句都必須用(),搭配limit使用???????

????????????????????????????????????????????????(select * from student where sex='male' order by age asc limit 100) union (select?*??????? ????????????????????????????????????????????????????from student where sex='female' order by age desc limit 100);

??????????????????????????????????????????????? 2.查詢多張表多張表的結構完全一樣，保存的數據(結構)也一樣

二.子查詢：一個查詢結果作為另一個查詢的條件

??????? 1.子查詢分類：按位置分類、結果分類

??????????????????? 1)：按位置查詢：子查詢(select語句)在外部查詢(select語句)中出現的位置。

???????????????????????????????????? 1.from子查詢：子查詢在from之后

???????????????????????????????????? 2.where子查詢：子查詢在where之后

???????????????????????????????????? 3.exists子查詢：子查詢在exists里面

??????????????????? 2)：按照結構子查詢：根據子查詢得到的結果進行分類

???????????????????????????????????? 1.標量子查詢：子查詢得到的結果是一行一列,例如一致課程名稱查詢id

?????????????????????????????????????????? select * from student where id=(select id from student where course='Java');
?????????????????????????????????????????? +----+--------+------+------+--------+
????????????????????????????????????????????| id | name?? | sex? | age? | course |
?????????????????????????????????????????? +----+--------+------+------+--------+
????????????????????????????????????????????|? 1 | steven | male |?? 20 | Java?? |
?????????????????????????????????????????? +----+--------+------+------+--------+

???????????????????????????????????? 2.列子查詢：子查詢得到的結果是一列多行，需要使用in作為查詢條件，類似的還有all、some、any

????????????????????????????????????????????select * from student where id =some(select id from student);

???????????????????????????????????????????? 使用all、some、any時必須在前面加=號

????????????????????????????????????????????select * from student where id in(select id from student);

??????????????????????????????????????????? +----+--------+--------+------+--------+
???????????????????????????????????????????? | id | name?? | sex??? | age? | course |
??????????????????????????????????????????? +----+--------+--------+------+--------+
???????????????????????????????????????????? |? 1 | steven | male?? |?? 20 | Java?? |
???????????????????????????????????????????? |? 2 | mary?? | female |?? 27 | PHP??? |
???????????????????????????????????????????? |? 3 | candy? | female |?? 24 | C語言? |
???????????????????????????????????????????? |? 4 | mark?? | male?? |?? 22 | C++??? |
???????????????????????????????????????????? |? 5 | hary?? | female |?? 26 | JavaEE |
????????????????????????????????????????????+----+--------+--------+------+--------+

???????????????????????????????????? 3.行子查詢：子查詢得到的結果時多行一列(多行多列),需要構造行元素，它由多個字段構成

??????????????????????????????????? select * from student where id=(select max(id) from student) or age=(select max(age) from

????????????????????????????????????student);

?????????????????????????????????????select * from student where (id,age)=(select max(id),max(age) from student);

???????????????????????????????????? 4.表子查詢：子查詢得到的結果是多行多列(位置在from之后)，返回結果當作二維表來使用

??????????????????????????????????? ?select * from (select * from student order by height desc) as stu group by sex;

???????????????????????????????????? 5.exists子查詢：用來判斷某些查詢條件是否滿足(跨表查詢),exists實在where之后返回結果0和1

????????????????????????????????????????select * from student where exists (select * from my_class);

總結

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