分頁

limit 開始位置, 查詢數目 或者 limit 查詢數目

       -- 限制查詢出來的資料個數

       select * from students where gender=1 limit 2;

       -- 查詢前5個數據

       select * from students limit 0, 5;

       -- 查詢id6-10（包含）的書序

       select * from students limit 5, 5;

       -- 每頁顯示2個，顯示第6頁的資訊, 按照年齡從小到大排序

       -- 失敗select * from students limit 2*(6-1),2;

       -- 失敗select * from students limit 10,2 order by age asc;

       select * from students order by age asc limit 10,2;

       select * from students where gender=2 order by height desc limit 0,2;

連線查詢

inner join ... on等同於 join…on

       -- select ... from 表A inner join 表B;

       select * from students inner join classes;

       -- 查詢 有能夠對應班級的學生以及班級資訊

       select * from students inner join classes on students.cls_id=classes.id;

       -- 按照要求顯示姓名、班級

       select students.*, classes.name from students inner join classes on students.cls_id=classes.id;

       select students.name, classes.name from students inner join classes on students.cls_id=classes.id;

       -- 給資料表起名字

       select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;

       -- 查詢 有能夠對應班級的學生以及班級資訊，顯示學生的所有資訊，只顯示班級名稱

       select s.*, c.name from students as s inner join classes as c on s.cls_id=c.id;

       -- 在以上的查詢中，將班級姓名顯示在第1列

       select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id;

       -- 查詢 有能夠對應班級的學生以及班級資訊, 按照班級進行排序

       -- select c.xxx s.xxx from student as s inner join clssses as c on .... order by ....;

       select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;

       -- 當時同一個班級的時候，按照學生的id進行從小到大排序

       select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;

left join… on

       -- 查詢每位學生對應的班級資訊

       select * from students as s left join classes as c on s.cls_id=c.id;

       -- 查詢沒有對應班級資訊的學生

       -- select ... from xxx as s left join xxx as c on..... where .....

       -- select ... from xxx as s left join xxx as c on..... having .....

       select * from students as s left join classes as c on s.cls_id=c.id having c.id is null;

       select * from students as s left join classes as c on s.cls_id=c.id where c.id is null;

right  join…. on

       -- 將資料表名字互換位置，用left join完成

自關聯

對於一些省市資訊，如何設計自關聯表將他們關聯起來呢？

對一個省市，我們做如下表示：

id（省市編號）  atitle（省市名稱）  pid（直接所屬上級城市的編號id）

因為省沒有所屬的省份，所以可以填寫為null，城市所屬的省份pid，填寫省所對應的編號id

這就是自關聯，表中的某一行，關聯了這個表中的另外一行

       -- 查詢所有省份

       select * from areas where pid is null;

       -- 查詢出山東省有哪些市

       select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle="山東省";

       -- 查詢出青島市有哪些縣城

       select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle="青島市";

       select * from areas where pid=(select aid from areas where atitle="青島市")

子查詢

       -- 標量子查詢

       -- 查詢出高於平均身高的資訊

       -- 查詢最高的男生資訊

       select * from students where height = 188;

       select * from students where height = (select max(height) from students);

       -- 列級子查詢

       -- 查詢學生的班級號能夠對應的學生資訊

       -- select * from students where cls_id in (select id from classes);