MySQL 表连接

时间:2022-05-23 我是小白呀人气:0

概述

从今天开始, 小白我将带领大家一起来补充一下数据库的知识.

表连接

表连接 (JOIN) 是在多个表之间通过连接条件. 使表之间发生关联, 进而能从多个表之间获取数据.

表连接语法:

SELECT 表名1.列名, 表名2.列名 FROM 表1, 表2 WHERE 条件

表连接的几种方式:

内连接: join 或者 inner join

自连接: 同一张表内的连接

外连接:

左外连接, left join
右外连接, right join
全外连接, full join

数据 (table1):

数据 (table2):

内连接

内连接是从结果表中删除与其他被连接表中没有匹配的所有行. (交集)

格式:

SELECT A.c1, B.c2 FROM A JOIN B ON A.c3 = B.c3

例子:

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "数据库链接成功\n";
} else {
    echo mysqli_connect_error();
}

# SQL语句
$SQL = "SELECT table1.name, table1.age, table2.class
        FROM table1 JOIN table2
        ON table1.name = table2.name";

# 执行
$result = mysqli_query($conn, $SQL);

# 查看是否执行成功
if ($result) {
    echo "SQL 语句执行成功!\n";
}else {
    echo mysqli_error($conn);
}

# 调试输出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 关闭连接
mysqli_close($conn);

?>

输出结果:

数据库链接成功
SQL 语句执行成功!
Array
(
[name] => 杨富贵
[age] => 18
[class] => 数学课
)
Array
(
[name] => 杨美丽
[age] => 19
[class] => 语文课
)
Array
(
[name] => 杨建国
[age] => 20
[class] => 英语课
)

左连接

包含做表的全部行 (不管右表是否存在与之匹配的行), 以及右表中全部匹配的行.

格式:

SELECT A.c1, B.c2 FROM A LEFT JOIN B ON A.c3 = B.c3

例子:

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "数据库链接成功\n";
} else {
    echo mysqli_connect_error();
}

# SQL语句
$SQL = "SELECT table1.name, table1.age, table2.class
        FROM table1 LEFT JOIN table2
        ON table1.name = table2.name";

# 执行
$result = mysqli_query($conn, $SQL);

# 查看是否执行成功
if ($result) {
    echo "SQL 语句执行成功!\n";
}else {
    echo mysqli_error($conn);
}

# 调试输出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 关闭连接
mysqli_close($conn);

?>

输出结果:

数据库链接成功
SQL 语句执行成功!
Array
(
[name] => 杨富贵
[age] => 18
[class] => 数学课
)
Array
(
[name] => 杨美丽
[age] => 19
[class] => 语文课
)
Array
(
[name] => 杨建国
[age] => 20
[class] => 英语课
)
Array
(
[name] => 杨铁柱
[age] => 21
[class] =>
)

右连接

包含右表的全部行 (不管左表是否存在与之匹配的行), 以及左表中全部匹配的行.

格式:

SELECT A.c1, B.c2 FROM A RIGHT JOIN B ON A.c3 = B.c3

例子:

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "数据库链接成功\n";
} else {
    echo mysqli_connect_error();
}

# SQL语句
$SQL = "SELECT table1.name, table1.age, table2.class
        FROM table1 RIGHT JOIN table2
        ON table1.name = table2.name";

# 执行
$result = mysqli_query($conn, $SQL);

# 查看是否执行成功
if ($result) {
    echo "SQL 语句执行成功!\n";
}else {
    echo mysqli_error($conn);
}

# 调试输出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 关闭连接
mysqli_close($conn);

?>

输出结果:

数据库链接成功
SQL 语句执行成功!
Array
(
[name] => 杨富贵
[age] => 18
[class] => 数学课
)
Array
(
[name] => 杨美丽
[age] => 19
[class] => 语文课
)
Array
(
[name] => 杨建国
[age] => 20
[class] => 英语课
)
Array
(
[name] =>
[age] =>
[class] => 历史课
)

全连接

包含左右两个表的全部行 (不管在另一个表中是否存在与之匹配的行).

格式:

SELECT A.c1, B.c2 FROM A LEFT JOIN B ON A.c3 = B.c3
UNION
SELECT A.c1, B.c2 FROM A RIGHT JOIN B ON A.c3 = B.c3

例子:

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "数据库链接成功\n";
} else {
    echo mysqli_connect_error();
}

# SQL语句
$SQL = "SELECT table1.name, table1.age, table2.class
        FROM table1 LEFT JOIN table2 ON table1.name = table2.name
        UNION 
        SELECT table1.name, table1.age, table2.class
        FROM table1 RIGHT JOIN table2 ON table1.name = table2.name";

# 执行
$result = mysqli_query($conn, $SQL);

# 查看是否执行成功
if ($result) {
    echo "SQL 语句执行成功!\n";
}else {
    echo mysqli_error($conn);
}

# 调试输出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 关闭连接
mysqli_close($conn);

?>

输出结果:

数据库链接成功
SQL 语句执行成功!
Array
(
[name] => 杨富贵
[age] => 18
[class] => 数学课
)
Array
(
[name] => 杨美丽
[age] => 19
[class] => 语文课
)
Array
(
[name] => 杨建国
[age] => 20
[class] => 英语课
)
Array
(
[name] => 杨铁柱
[age] => 21
[class] =>
)
Array
(
[name] =>
[age] =>
[class] => 历史课
)

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