MySQL學習（四） SQL鏈接查詢

時間 2019-12-09

標籤 mysql 學習 sql 鏈接查詢欄目 MySQL 简体版

原文原文鏈接

更多狀況下，咱們查詢的數據來源於多張表，全部有必要了解一下MySQL中的鏈接查詢。mysql

SQL中將鏈接查詢分紅四類：交叉鏈接，內鏈接，外鏈接和天然鏈接。sql

數據準備blog

student表ci

-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(10) DEFAULT NULL,
  `age` tinyint(4) DEFAULT NULL,
  `classId` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', 's1', '20', '1');
INSERT INTO `student` VALUES ('2', 's2', '22', '1');
INSERT INTO `student` VALUES ('3', 's3', '22', '2');
INSERT INTO `student` VALUES ('4', 's4', '22', null);

class表數學

-- ----------------------------
-- Table structure for `class`
-- ----------------------------
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `cname` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of class
-- ----------------------------
INSERT INTO `class` VALUES ('1', '一班');
INSERT INTO `class` VALUES ('2', '二班');
INSERT INTO `class` VALUES ('3', '');

score表class

-- ----------------------------
-- Table structure for `score`
-- ----------------------------
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  `score` decimal(4,1) DEFAULT NULL,
  `studentId` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of score
-- ----------------------------
INSERT INTO `score` VALUES ('1', '語文', '90.0', '1');
INSERT INTO `score` VALUES ('2', '數學', '95.0', '1');
INSERT INTO `score` VALUES ('3', '語文', '92.0', '2');
INSERT INTO `score` VALUES ('4', '數學', '88.0', '2');
INSERT INTO `score` VALUES ('5', '語文', '96.0', '3');
INSERT INTO `score` VALUES ('6', '數學', null, '3');

交叉鏈接

交叉鏈接(CROSS JOIN)是用左表中的每一行與右表中的每一行進行鏈接，不能使用ON關鍵字。所獲得的結果將是這兩個表中各行數據的全部組合，即這兩個表全部數據的笛卡爾積。若是A表有4條記錄，B表有3條，則結果有4*3=12條記錄。im

mysql> SELECT * FROM student CROSS JOIN class;
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  1 | s1   |  20 |       1 |  2 | 二班  |
|  1 | s1   |  20 |       1 |  3 |       |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  2 | 二班  |
|  2 | s2   |  22 |       1 |  3 |       |
|  3 | s3   |  22 |       2 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
|  3 | s3   |  22 |       2 |  3 |       |
|  4 | s4   |  22 | NULL    |  1 | 一班  |
|  4 | s4   |  22 | NULL    |  2 | 二班  |
|  4 | s4   |  22 | NULL    |  3 |       |
+----+------+-----+---------+----+-------+
12 rows in set

若是給交叉鏈接加上WHERE關鍵字，此時將返回符合條件的結果集，這時候與內鏈接的執行結果同樣。數據

mysql> SELECT * FROM student CROSS JOIN class WHERE student.classId = class.id;
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
+----+------+-----+---------+----+-------+
3 rows in set

內鏈接

內鏈接(INNER JOIN)是用左表中的每一行與右表中的全部記錄進行匹配，查詢的結果爲兩個表通過ON條件過濾後的笛卡爾積查詢

mysql> SELECT * FROM student INNER JOIN class ON student.classId = class.id; -- 推薦寫法，INNER可寫可不寫
+----+------+-----+---------+----+-------+
| id | name | age | classId | id | cname |
+----+------+-----+---------+----+-------+
|  1 | s1   |  20 |       1 |  1 | 一班  |
|  2 | s2   |  22 |       1 |  1 | 一班  |
|  3 | s3   |  22 |       2 |  2 | 二班  |
+----+------+-----+---------+----+-------+
3 rows in set

等價於di

mysql> SELECT * FROM student, class WHERE student.classId = class.id;
+----+------+-----+---------+----+------+
| id | name | age | classId | id | name |
+----+------+-----+---------+----+------+
|  1 | s1   |  20 |       1 |  1 | 一班 |
|  2 | s2   |  22 |       1 |  1 | 一班 |
|  3 | s3   |  22 |       2 |  2 | 二班 |
+----+------+-----+---------+----+------+
3 rows in set

能夠鏈接多張表

mysql> SELECT * FROM student
    -> JOIN class ON student.classId = class.id
    -> JOIN score ON student.id = score.studentId;
+----+------+-----+---------+----+-------+----+------+-------+-----------+
| id | name | age | classId | id | cname | id | name | score | studentId |
+----+------+-----+---------+----+-------+----+------+-------+-----------+
|  1 | s1   |  20 |       1 |  1 | 一班  |  1 | 語文 | 90    |         1 |
|  1 | s1   |  20 |       1 |  1 | 一班  |  2 | 數學 | 95    |         1 |
|  2 | s2   |  22 |       1 |  1 | 一班  |  3 | 語文 | 92    |         2 |
|  2 | s2   |  22 |       1 |  1 | 一班  |  4 | 數學 | 88    |         2 |
|  3 | s3   |  22 |       2 |  2 | 二班  |  5 | 語文 | 96    |         3 |
|  3 | s3   |  22 |       2 |  2 | 二班  |  6 | 數學 | NULL  |         3 |
+----+------+-----+---------+----+-------+----+------+-------+-----------+
6 rows in set

外鏈接

左外鏈接

左外鏈接包含LEFT JOIN左表全部行，若是左表中某行在右表沒有匹配，則結果中對應行右表的部分所有爲空(NULL)。

mysql> SELECT * FROM student LEFT JOIN class ON student.classId = class.id; -- 或者LEFT OUTER JOIN
+----+------+-----+---------+------+-------+
| id | name | age | classId | id   | cname |
+----+------+-----+---------+------+-------+
|  1 | s1   |  20 |       1 |    1 | 一班  |
|  2 | s2   |  22 |       1 |    1 | 一班  |
|  3 | s3   |  22 |       2 |    2 | 二班  |
|  4 | s4   |  22 | NULL    | NULL | NULL  |
+----+------+-----+---------+------+-------+
4 rows in set

右外鏈接

右外鏈接包含RIGHT JOIN左表全部行，若是右表中某行在左表沒有匹配，則結果中對應行左表的部分所有爲空(NULL)。

mysql> SELECT * FROM student RIGHT JOIN class ON student.classId = class.id; -- 或者RIGHT OUTER JOIN
+------+------+------+---------+----+-------+
| id   | name | age  | classId | id | cname |
+------+------+------+---------+----+-------+
|    1 | s1   |   20 |       1 |  1 | 一班  |
|    2 | s2   |   22 |       1 |  1 | 一班  |
|    3 | s3   |   22 |       2 |  2 | 二班  |
| NULL | NULL | NULL | NULL    |  3 |       |
+------+------+------+---------+----+-------+
4 rows in set

全外鏈接

若是左表中某行在右表沒有匹配，則結果中對應行右表的部分所有爲空(NULL)，若是右表中某行在左表沒有匹配，則結果中對應行左表的部分所有爲空(NULL)。MySQL不支持FULL JOIN，可是咱們能夠對左鏈接和右鏈接的結果作UNION操做來實現

mysql> SELECT * FROM student LEFT JOIN class ON student.classId = class.id
    -> UNION
    -> SELECT * FROM student RIGHT JOIN class ON student.classId = class.id;
+------+------+------+---------+------+-------+
| id   | name | age  | classId | id   | cname |
+------+------+------+---------+------+-------+
|    1 | s1   |   20 |       1 |    1 | 一班  |
|    2 | s2   |   22 |       1 |    1 | 一班  |
|    3 | s3   |   22 |       2 |    2 | 二班  |
|    4 | s4   |   22 | NULL    | NULL | NULL  |
| NULL | NULL | NULL | NULL    |    3 |       |
+------+------+------+---------+------+-------+
5 rows in set

天然鏈接

天然鏈接無需指定鏈接列，SQL會檢查兩個表中是否有相同名稱的列，且相同的列名稱只能有一個，天然鏈接基本不用。

mysql> SELECT * FROM student NATURAL JOIN class;
+----+------+-----+---------+-------+
| id | name | age | classId | cname |
+----+------+-----+---------+-------+
|  1 | s1   |  20 |       1 | 一班  |
|  2 | s2   |  22 |       1 | 二班  |
|  3 | s3   |  22 |       2 |       |
+----+------+-----+---------+-------+
3 rows in set

能夠看到只有一列id了，由於student與class兩張表中相同的id列自動合併了，至關於內鏈接

SELECT * FROM student INNER JOIN class ON student.id = class.id

若是更改class表的cname字段名稱爲name會出現什麼狀況呢？

mysql> ALTER TABLE class CHANGE cname name VARCHAR(10);
Query OK, 3 rows affected
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM student NATURAL JOIN class; -- 由於兩張表同名字段有兩個，全部結果爲空
Empty set

因爲如今student與class表有兩個同名的字段，全部結果爲空