MariaDB左外連接

MariaDB LEFT OUTER JOIN用于返回ON條件中指定的左側(cè)表中的所有行，并僅返回滿足連接條件的其他表中的行。

LEFT OUTER JOIN也被稱為LEFT JOIN。

語(yǔ)法：

SELECT columns  
FROM table1  
LEFT [OUTER] JOIN table2  
ON table1.column = table2.column;

圖形表示如下：

注： 上圖中，兩個(gè)圖形的左側(cè)表(table1)和右側(cè)表(table2)中間交叉藍(lán)色部分，以及左側(cè)表(table1)就是連接返回的結(jié)果集。

為了方便演示，我們需要?jiǎng)?chuàng)建兩個(gè)表，并插入一些數(shù)據(jù) -

USE testdb;
DROP table if exists students;
DROP table if exists subjects;
DROP table if exists scores;
-- 學(xué)生信息
CREATE TABLE students(  
    student_id INT NOT NULL AUTO_INCREMENT,  
    student_name VARCHAR(100) NOT NULL,  
    student_address VARCHAR(40) NOT NULL,  
    admission_date DATE,  
    PRIMARY KEY ( student_id )
);

-- 科目信息
CREATE TABLE subjects(  
    subject_id INT NOT NULL AUTO_INCREMENT,  
    subject_name VARCHAR(100) NOT NULL,
    PRIMARY KEY ( subject_id )
);

-- 成績(jī)信息
CREATE TABLE scores(  
    id INT NOT NULL AUTO_INCREMENT,
    student_id int(10) NOT NULL,
    subject_id int(10) NOT NULL,
    score float(4,1) DEFAULT NULL,
    created_time datetime DEFAULT NULL,
    PRIMARY KEY ( id )
);

插入數(shù)據(jù) -

--- 學(xué)生信息數(shù)據(jù)
INSERT INTO students  
(student_id, student_name, student_address, admission_date)  
VALUES(1,'Maxsu','Haikou','2017-01-07 00:00:00');
INSERT INTO students  
(student_id, student_name, student_address, admission_date)  
VALUES  
(2,'JMaster','Beijing','2016-05-07 00:00:00'),  
(3,'Mahesh','Guangzhou','2016-06-07 00:00:00'),  
(4,'Kobe','Shanghai','2016-02-07 00:00:00'),  
(5,'Blaba','Shenzhen','2016-08-07 00:00:00');

-- 科目信息數(shù)據(jù)
INSERT INTO subjects  
(subject_id, subject_name)  
VALUES(1,'計(jì)算機(jī)網(wǎng)絡(luò)基礎(chǔ)');

INSERT INTO subjects  
(subject_id, subject_name)  
VALUES(2,'高等數(shù)學(xué)');

INSERT INTO subjects  
(subject_id, subject_name)  
VALUES(3,'離散數(shù)學(xué)');

-- 分?jǐn)?shù)
INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(1,1,81,'2017-11-18 19:30:02');

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(1,2,89,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(1,3,92,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(2,2,95,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(2,3,72,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(3,1,59,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(3,3,77,NOW());

INSERT INTO scores  
(student_id, subject_id, score, created_time)  
VALUES(4,2,81,NOW());

當(dāng)前studens表中的行記錄如下 -

MariaDB [testdb]> select * from students;
+------------+--------------+-----------------+----------------+
| student_id | student_name | student_address | admission_date |
+------------+--------------+-----------------+----------------+
|          1 | Maxsu        | Haikou          | 2017-01-07     |
|          2 | JMaster      | Beijing         | 2016-05-07     |
|          3 | Mahesh       | Guangzhou       | 2016-06-07     |
|          4 | Kobe         | Shanghai        | 2016-02-07     |
|          5 | Blaba        | Shenzhen        | 2016-08-07     |
+------------+--------------+-----------------+----------------+
5 rows in set (0.00 sec)

當(dāng)前score表中的行記錄如下 -

MariaDB [testdb]> select * from scores;
+----+------------+------------+-------+---------------------+
| id | student_id | subject_id | score | created_time        |
+----+------------+------------+-------+---------------------+
|  1 |          1 |          1 |  81.0 | 2017-11-18 19:30:02 |
|  2 |          1 |          2 |  89.0 | 2017-11-28 22:31:57 |
|  3 |          1 |          3 |  92.0 | 2017-11-28 22:31:58 |
|  4 |          2 |          2 |  95.0 | 2017-11-28 22:31:58 |
|  5 |          2 |          3 |  72.0 | 2017-11-28 22:31:58 |
|  6 |          3 |          1 |  59.0 | 2017-11-28 22:31:58 |
|  7 |          3 |          3 |  77.0 | 2017-11-28 22:31:58 |
|  8 |          4 |          2 |  81.0 | 2017-11-28 22:31:58 |
+----+------------+------------+-------+---------------------+
8 rows in set (0.00 sec)

示例1

使用以下語(yǔ)法根據(jù)給定的參數(shù)條件連接兩個(gè)表 - students和scores，即查詢學(xué)生信息和對(duì)應(yīng)的成績(jī)信息，如果沒(méi)有成績(jī)則使用NULL值表示。

SELECT students.student_id, students.student_name, scores.subject_id, scores.score  
FROM students   
LEFT JOIN scores  
ON students.student_id = scores.student_id
ORDER BY students.student_id;

上面查詢語(yǔ)句查詢所有科目的考試分?jǐn)?shù)，得到以下結(jié)果 -

MariaDB [testdb]> SELECT students.student_id, students.student_name, scores.subject_id, scores.score
    -> FROM students
    -> LEFT JOIN scores
    -> ON students.student_id = scores.student_id
    -> ORDER BY students.student_id;
+------------+--------------+------------+-------+
| student_id | student_name | subject_id | score |
+------------+--------------+------------+-------+
|          1 | Maxsu        |          1 |  81.0 |
|          1 | Maxsu        |          2 |  89.0 |
|          1 | Maxsu        |          3 |  92.0 |
|          2 | JMaster      |          2 |  95.0 |
|          2 | JMaster      |          3 |  72.0 |
|          3 | Mahesh       |          1 |  59.0 |
|          3 | Mahesh       |          3 |  77.0 |
|          4 | Kobe         |          2 |  81.0 |
|          5 | Blaba        |       NULL |  NULL |
+------------+--------------+------------+-------+
9 rows in set (0.00 sec)

上面示例的查詢結(jié)果中，由于最后一行(student_id=5)的學(xué)生還沒(méi)有任何信息，所以在使用LEFT JOIN連接后，右側(cè)表(scores)相關(guān)列的值使用NULL來(lái)填充。

示例2

查詢指定學(xué)生，并且成績(jī)大于85分的信息 -

SELECT students.student_id, students.student_name, scores.subject_id, scores.score  
FROM students   
LEFT JOIN scores  
ON students.student_id = scores.student_id
WHERE students.student_name='Maxsu' AND scores.score > 85;

執(zhí)行上面查詢語(yǔ)句，得到以下結(jié)果 -

MariaDB [testdb]> SELECT students.student_id, students.student_name, scores.subject_id, scores.score
    -> FROM students
    -> LEFT JOIN scores
    -> ON students.student_id = scores.student_id
    -> WHERE students.student_name='Maxsu' AND scores.score > 85;
+------------+--------------+------------+-------+
| student_id | student_name | subject_id | score |
+------------+--------------+------------+-------+
|          1 | Maxsu        |          2 |  89.0 |
|          1 | Maxsu        |          3 |  92.0 |
+------------+--------------+------------+-------+
2 rows in set (0.00 sec)

示例3

查詢沒(méi)有考試成績(jī)的學(xué)生信息(尚未錄入) -

SELECT students.student_id, students.student_name, scores.subject_id, scores.score  
FROM students   
LEFT JOIN scores  
ON students.student_id = scores.student_id
WHERE scores.score IS NULL;

執(zhí)行上面查詢語(yǔ)句，得到以下結(jié)果 -

MariaDB [testdb]> SELECT students.student_id, students.student_name, scores.subject_id, scores.score
    -> FROM students
    -> LEFT JOIN scores
    -> ON students.student_id = scores.student_id
    -> WHERE scores.score IS NULL;
+------------+--------------+------------+-------+
| student_id | student_name | subject_id | score |
+------------+--------------+------------+-------+
|          5 | Blaba        |       NULL |  NULL |
+------------+--------------+------------+-------+
1 row in set (0.00 sec)