分頁、連線查詢、自關聯、子查詢
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);