MySQL數據庫學習之去重與連接查詢詳解

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1.去重

示例表內容參考此文章

有些 MySQL 數據表中可能存在重復的記錄，有些情況我們允許重復數據的存在，但有時候我們也需要刪除這些重復的數據。

例如：去重顯示崗位信息：

mysql> select distinct job from emp;
+-----------+
| job       |
+-----------+
| CLERK     |
| SALESMAN  |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
+-----------+
5 rows in set (0.02 sec)

另一個示例：聯合去重，查找部門和崗位的獨有信息：

mysql> select distinct job,deptno from emp;
+-----------+--------+
| job       | deptno |
+-----------+--------+
| CLERK     |     20 |
| SALESMAN  |     30 |
| MANAGER   |     20 |
| MANAGER   |     30 |
| MANAGER   |     10 |
| ANALYST   |     20 |
| PRESIDENT |     10 |
| CLERK     |     30 |
| CLERK     |     10 |
+-----------+--------+
9 rows in set (0.00 sec)

另一個示例：現在我們想統計一下工作崗位的數量，結合使用count函數：

mysql> select count(distinct job) from emp;
+---------------------+
| count(distinct job) |
+---------------------+
|                   5 |
+---------------------+
1 row in set (0.00 sec)

2.連接查詢

我們已經學會瞭如何在一張表中讀取數據，這是相對簡單的，但是在真正的應用中經常需要從多個數據表中讀取數據。

JOIN 按照功能大致分為如下三類：

INNER JOIN（內連接,或等值連接）：獲取兩個表中字段匹配關系的記錄。

LEFT JOIN（左連接）：獲取左表所有記錄，即使右表沒有對應匹配的記錄。

RIGHT JOIN（右連接）： 與 LEFT JOIN 相反，用於獲取右表所有記錄，即使左表沒有對應匹配的記錄。

多表連接的機制是：從其中一個表中取出每一條數據，從另一個表中的數據行進行匹配。這就涉及到瞭效率控制問題

使用where進行多表連接查詢

現在我們來演示一個例子：取出每個員工的名字和部門名字：

mysql> select ename,dname
    -> from emp,dept
    -> where emp.deptno = dept.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)

上面的sql語句實際上效率很低，我們嘗試進行優化（給表起別名）：（sql92語法）

mysql> select e.ename,d.dname
    -> from emp e,dept d
    -> where e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)

註意：表的連接次數越多，效率越低，請盡量減少表的連接次數！

內連接 – 等值連接

還是上面的例子，取出每個員工的名字和部門名字：（sql99語法）

內連接，我們使用inner

mysql> select e.ename,d.dname
    -> from emp e
    -> inner join
    -> dept d
    -> on
    -> e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
14 rows in set (0.00 sec)

sql99的優點是：表的連接是獨立的，不占用where的位置。使sql語句整體更加清晰

內連接 – 非等值連接

案例：找出每個員工的薪資等級，要求顯示員工名，薪資，薪資等級

mysql> select
    -> e.ename,e.sal,s.grade
    -> from
    -> emp e
    -> inner join
    -> salgrade s
    -> on
    -> e.sal between s.losal and s.hisal;
+--------+---------+-------+
| ename  | sal     | grade |
+--------+---------+-------+
| SMITH  |  800.00 |     1 |
| ALLEN  | 1600.00 |     3 |
| WARD   | 1250.00 |     2 |
| JONES  | 2975.00 |     4 |
| MARTIN | 1250.00 |     2 |
| BLAKE  | 2850.00 |     4 |
| CLARK  | 2450.00 |     4 |
| SCOTT  | 3000.00 |     4 |
| KING   | 5000.00 |     5 |
| TURNER | 1500.00 |     3 |
| ADAMS  | 1100.00 |     1 |
| JAMES  |  950.00 |     1 |
| FORD   | 3000.00 |     4 |
| MILLER | 1300.00 |     2 |
+--------+---------+-------+
14 rows in set (0.01 sec)

內連接 – 自連接

案例：查詢員工的上級領導，要求顯示員工名和對應的領導名

我們可以發現，員工和領導的關系在一張表中，此時需要用到自連接（技巧：一張表看成兩張表）

mysql> select
    -> a.ename as '員工名',b.ename as '領導名'
    -> from emp a
    -> join emp b
    -> on
    -> a.mgr = b.empno;
+-----------+-----------+
| 員工名    | 領導名      |
+-----------+-----------+
| SMITH     | FORD      |
| ALLEN     | BLAKE     |
| WARD      | BLAKE     |
| JONES     | KING      |
| MARTIN    | BLAKE     |
| BLAKE     | KING      |
| CLARK     | KING      |
| SCOTT     | JONES     |
| TURNER    | BLAKE     |
| ADAMS     | SCOTT     |
| JAMES     | BLAKE     |
| FORD      | JONES     |
| MILLER    | CLARK     |
+-----------+-----------+
13 rows in set (0.00 sec)

外連接 – 左右外連接

外連接與內連接的區別是，外連接沒有匹配成功的某一個表的記錄也會被取出

案例：查找員工的部門信息。要求部門即使沒有員工也要查出

mysql> select
    -> e.ename,d.dname
    -> from emp e
    -> right join dept d
    -> on
    -> e.deptno = d.deptno;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
| NULL   | OPERATIONS |
+--------+------------+
15 rows in set (0.00 sec)

同樣的，如果是左外連接，將查詢出左表的全部數據，使用left join關鍵字即可

外連接的查詢結果條數一定是 >= 內連接的查詢結果條數

三表連接

更為復雜的情況是，群表連接

我們來看一個案例：

找出每個員工的部門名稱及工資等級。要求顯示員工名，部門名，薪資，薪資等級

mysql> select
    -> e.ename,e.sal,d.dname,s.grade
    -> from emp e
    -> join dept d
    -> on e.deptno = d.deptno
    -> join salgrade s
    -> on e.sal between s.losal and s.hisal;
+--------+---------+------------+-------+
| ename  | sal     | dname      | grade |
+--------+---------+------------+-------+
| SMITH  |  800.00 | RESEARCH   |     1 |
| ALLEN  | 1600.00 | SALES      |     3 |
| WARD   | 1250.00 | SALES      |     2 |
| JONES  | 2975.00 | RESEARCH   |     4 |
| MARTIN | 1250.00 | SALES      |     2 |
| BLAKE  | 2850.00 | SALES      |     4 |
| CLARK  | 2450.00 | ACCOUNTING |     4 |
| SCOTT  | 3000.00 | RESEARCH   |     4 |
| KING   | 5000.00 | ACCOUNTING |     5 |
| TURNER | 1500.00 | SALES      |     3 |
| ADAMS  | 1100.00 | RESEARCH   |     1 |
| JAMES  |  950.00 | SALES      |     1 |
| FORD   | 3000.00 | RESEARCH   |     4 |
| MILLER | 1300.00 | ACCOUNTING |     2 |
+--------+---------+------------+-------+
14 rows in set (0.00 sec)

再來看一個更復雜的情況：

找出每個員工的部門名稱及工資等級及領導名稱。要求顯示員工名，部門名，領導名，薪資，薪資等級

mysql> select
    -> e.ename,e.sal,d.dname,s.grade,l.ename
    -> from emp e
    -> join dept d
    -> on e.deptno = d.deptno
    -> join salgrade s
    -> on e.sal between s.losal and s.hisal
    -> left join
    -> emp l
    -> on e.mgr = l.empno;
+--------+---------+------------+-------+-------+
| ename  | sal     | dname      | grade | ename |
+--------+---------+------------+-------+-------+
| SMITH  |  800.00 | RESEARCH   |     1 | FORD  |
| ALLEN  | 1600.00 | SALES      |     3 | BLAKE |
| WARD   | 1250.00 | SALES      |     2 | BLAKE |
| JONES  | 2975.00 | RESEARCH   |     4 | KING  |
| MARTIN | 1250.00 | SALES      |     2 | BLAKE |
| BLAKE  | 2850.00 | SALES      |     4 | KING  |
| CLARK  | 2450.00 | ACCOUNTING |     4 | KING  |
| SCOTT  | 3000.00 | RESEARCH   |     4 | JONES |
| KING   | 5000.00 | ACCOUNTING |     5 | NULL  |
| TURNER | 1500.00 | SALES      |     3 | BLAKE |
| ADAMS  | 1100.00 | RESEARCH   |     1 | SCOTT |
| JAMES  |  950.00 | SALES      |     1 | BLAKE |
| FORD   | 3000.00 | RESEARCH   |     4 | JONES |
| MILLER | 1300.00 | ACCOUNTING |     2 | CLARK |
+--------+---------+------------+-------+-------+
14 rows in set (0.00 sec)

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