MySQL(9) 之 DQL查詢(多表查詢)
多表查詢
含義:當查詢的欄位來自多個表的時候,就會用到多表查詢
笛卡爾迪 現象:表1 有n行,表2有m行,結果m*n行;
發生原因:沒有有效的連線條件
分類:
內連線:
等值連線
非等值連線
自連線
外連結:
左外連線
右外連線
全外連線
交叉連線:
92 語法:
SELECT * FROM beauty;
SELECT * FROM boys;
SELECT NAME,boyName FROM boys,beauty
WHERE beauty.`boyfriend_id`=boys.`id`;
# 1. 等值連線
特點:1.多表等值連線的結果為多表的交集部分
2.n表連線,至少需要n-1個連線條件
3.多表的順序沒有要求
4.一般需要為表起別名;
SELECT NAME,boyName
FROM boys,beauty
WHERE beauty.boyfriend_id=boys.id;
# 查詢有獎金的員工名和部門名
SELECT last_name ,department_name
FROM employees e,departments d
WHERE e.`department_id`=d.`department_id`
AND e.`commission_pct` IS NOT NULL;
# 查詢城市名中第二個字元為o的部門名和城市名
SELECT department_name, city
FROM departments d,locations l
WHERE d.`location_id`=l.`location_id`
AND city LIKE "_o%";
#查詢每個城市的部門個數
SELECT COUNT(*) ,city
FROM departments d,locations l
WHERE d.`location_id`=l.`location_id`
GROUP BY city;
# 查詢每個工種的名成和員工的個數,並按員工個數降序
SELECT job_title,COUNT(*)
FROM employees e,jobs j
WHERE e.`job_id`=j.`job_id`
GROUP BY job_title
ORDER BY COUNT(*) DESC;
# 查詢 員工名、部門名和所在的城市
SELECT last_name ,department_name,city
FROM employees e,departments d,locations l
WHERE e.`department_id`=d.`department_id`
AND d.`location_id`=l.`location_id`
多表查詢
# 多表查詢的非等值連線
SELECT salary ,grade_level
FROM employees e,job_grades j
WHERE salary BETWEEN j.`lowest_sal` AND j.`highest_sal`
AND j.grade_level ='A';
# 自連線
#查詢員工名和上級名稱
SELECT e.`last_name`,m.`last_name`
FROM employees e,employees m
WHERE e.`manager_id`=m.`employee_id`;
99語法
# 1. 內連線
語法:
select 列名
from 表1 別名
inner join 表2 別名
on 連線條件;
分類:等值,非等值,自連線
特點:
1.可以新增排序,分組,篩選
2.inner'可以省略
3. 篩選條件方法where後面,連線條件放在on後面,提高閱讀
# 等值連線
# 查詢員工名和部門名
SELECT last_name ,department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`;
# 查詢名字中包含e的員工名和工種名
SELECT last_name ,job_title
FROM employees e
INNER JOIN jobs j
ON e.`job_id`=j.`job_id`
WHERE e.`last_name` LIKE '%e%';
# 查詢部門個數>3 的城市名和部門個數
SELECT city,COUNT(*)
FROM departments d
INNER JOIN locations l
ON d.`location_id`=l.`location_id`
GROUP BY city
HAVING COUNT(*)>3;
# 查詢那個部門的員工個數》3的部門名和員工個數,並按照個數降序
SELECT department_name,COUNT(*)
FROM departments d
INNER JOIN employees e
ON e.`department_id`=d.`department_id`
GROUP BY department_name
HAVING COUNT(*)>3
ORDER BY COUNT(*) DESC;
# 查詢員工名,部門名,工種名,並按照部門名降序
SELECT last_name ,department_name ,job_title
FROM employees e
INNER JOIN departments d ON e.`department_id`=d.`department_id`
INNER JOIN jobs j ON e.`job_id`=j.`job_id`
ORDER BY department_name DESC;
# 非等值連線
# 查詢員工工資的級別
SELECT salary ,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;
# 查詢工資級別的個數>20 的個數,並且按照工資級別降序
SELECT COUNT(*) ,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`
GROUP BY grade_level
HAVING COUNT(*)>20
ORDER BY grade_level DESC;
# 自連線
#查詢員工的名字和上級的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`;
#查詢員工名包含k的員工名和上級的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`
WHERE e.`last_name` LIKE "%k%";
外連結
應用場景:用於查詢一個表中有,另一個表中沒有的記錄
特點:
1.外連結查詢的結果為主表中的所有記錄
如果從表中和它匹配的,則顯示匹配的值
如果從表中沒有匹配到,則顯示null
外連線查詢結果=內連線結果+主表中有而從表中沒有的記錄
2.左外連線,left join 左邊的是主表
右外連線,right join 右邊的是主表
3.左外和右外交換兩個表的順序,可以實現同樣的效果
4.全外連線,內連線的結果+表1中有但是表2中沒有的+表2中有但是表1中沒有的;
# 引入:查詢男朋友不在男神表中的女神名
# 左外連線
SELECT b.`name`,bo.`boyName`
FROM beauty b
LEFT OUTER JOIN boys bo
ON b.`boyfriend_id`=bo.`id`
WHERE bo.`id` IS NULL;
# 左外連線
SELECT b.`name`,bo.`boyName`
FROM boys bo
RIGHT OUTER JOIN beauty b
ON b.`boyfriend_id`=bo.`id`
WHERE bo.`id` IS NULL;
# 查詢那個部門沒有員工
# 左外
SELECT d.* ,e.*
FROM departments d
LEFT OUTER JOIN employees e
ON d.`department_id`=e.`department_id`
WHERE e.`employee_id` IS NULL;
# 全外連線
SELECT b.*,bo.*
FROM beauty b
FULL OUTER JOIN boys bo
ON b.`boyfriend_id`= bo.id;
交叉連線
# 交叉連線(笛卡爾乘積)
SELECT b.* ,bo.*
FROM beauty b
CROSS JOIN boys bo;