在两个表上执行MySQL LEFT JOIN？

2024-05-17 14:12:03 16

让我们首先创建两个表，并用外键约束将它们联接起来。创建第一个表的查询如下-

mysql> create table ParentTable
   -> (
   -> UniqueId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> EmployeeName varchar(10)
   -> );

使用insert命令在第一个表中插入一些记录。查询如下-

mysql> insert into ParentTable(EmployeeName) values('John');
mysql> insert into ParentTable(EmployeeName) values('Carol');
mysql> insert into ParentTable(EmployeeName) values('Sam');
mysql> insert into ParentTable(EmployeeName) values('Bob');

现在，您可以使用select语句显示表中的所有记录。查询如下-

mysql> select *from ParentTable;

以下是输出-

+----------+--------------+
| UniqueId | EmployeeName |
+----------+--------------+
|        1 | John         |
|        2 | Carol        |
|        3 | Sam          |
|        4 | Bob          |
+----------+--------------+
4 rows in set (0.00 sec)

创建具有外键约束的第二张表的查询如下-

mysql> create table ChildTable
   -> (
   -> UniqueId int NOT NULL PRIMARY KEY,
   -> EmployeeAddress varchar(100),
   -> CONSTRAINT fk_uniqueId FOREIGN KEY(UniqueId) references ParentTable(UniqueId)
   -> );

现在，使用insert命令在第二个表中插入一些记录。查询如下-

mysql> insert into ChildTable values(1,'15 West Shady Lane Starkville, MS 39759');
mysql> insert into ChildTable values(2,'72 West Rock Creek St. Oxford, MS 38655');
mysql> insert into ChildTable(UniqueId) values(3);
mysql> insert into ChildTable values(4,'119 North Sierra St. Marysville, OH 43040');

使用select语句显示表中的所有记录。查询如下-

mysql> select *from ChildTable;

以下是输出-

+----------+-------------------------------------------+
| UniqueId | EmployeeAddress                           |
+----------+-------------------------------------------+
|        1 | 15 West Shady Lane Starkville, MS 39759   |
|        2 | 72 West Rock Creek St. Oxford, MS 38655   |
|        3 | NULL                                      |
|        4 | 119 North Sierra St. Marysville, OH 43040 |
+----------+-------------------------------------------+
4 rows in set (0.00 sec)

现在让我们使用左联接来联接表。查询如下-

mysql> select
ParentTable.UniqueId,ParentTable.EmployeeName,ChildTable.EmployeeAddress from ParentTable left join
   -> ChildTable on ParentTable.UniqueId=ChildTable.UniqueId;

以下是输出-

+----------+--------------+-------------------------------------------+
| UniqueId | EmployeeName | EmployeeAddress                           |
+----------+--------------+-------------------------------------------+
|        1 | John         | 15 West Shady Lane Starkville, MS 39759   |
|        2 | Carol        | 72 West Rock Creek St. Oxford, MS 38655   |
|        3 | Sam          | NULL                                      |
|        4 | Bob          | 119 North Sierra St. Marysville, OH 43040 |
+----------+--------------+-------------------------------------------+
4 rows in set (0.00 sec)

在两个表上执行MySQL LEFT JOIN？

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