问一个update的sql

-- data
id     id_emp     emp_make
1        1           小明
2        2           小明
3        2           中明
4                    大明
5                    无明
-- employee
id  emp        
1   小明
2   中明
3   大明

首先
data表的emp_make不能为null
id_emp和employee表的id关联
我现在想将data表中id_emp为null和''，并且emp_make在employee表中也能找到对应的记录的时候
就将id_emp更新为employee表中中对应的id

也就是最终data表中的结果将变成

id      id_emp     emp_make
1        1           小明
2        2           小明
3        2           中明
4        3           大明
5                    无明

下面是建表语句

-- ----------------------------
-- Table structure for [dbo].[employee]
-- ----------------------------
DROP TABLE [dbo].[employee]
GO
CREATE TABLE [dbo].[employee] (
[id] int NOT NULL ,
[emp] nvarchar(10) NULL 
)


GO

-- ----------------------------
-- Records of employee
-- ----------------------------
INSERT INTO [dbo].[employee] ([id], [emp]) VALUES (N'1', N'小明');
GO
INSERT INTO [dbo].[employee] ([id], [emp]) VALUES (N'2', N'中明');
GO
INSERT INTO [dbo].[employee] ([id], [emp]) VALUES (N'3', N'大明');
GO

-- ----------------------------
-- Indexes structure for table employee
-- ----------------------------

-- ----------------------------
-- Primary Key structure for table [dbo].[employee]
-- ----------------------------
ALTER TABLE [dbo].[employee] ADD PRIMARY KEY ([id])
GO

-- ----------------------------
-- Table structure for [dbo].[data]
-- ----------------------------
DROP TABLE [dbo].[data]
GO
CREATE TABLE [dbo].[data] (
[id] int NOT NULL ,
[id_emp] int NULL ,
[emp_make] nvarchar(10) NULL 
)


GO

-- ----------------------------
-- Records of data
-- ----------------------------
INSERT INTO [dbo].[data] ([id], [id_emp], [emp_make]) VALUES (N'1', N'1', N'小明');
GO
INSERT INTO [dbo].[data] ([id], [id_emp], [emp_make]) VALUES (N'2', N'2', N'小明');
GO
INSERT INTO [dbo].[data] ([id], [id_emp], [emp_make]) VALUES (N'3', N'2', N'小明');
GO
INSERT INTO [dbo].[data] ([id], [id_emp], [emp_make]) VALUES (N'4', null, N'大明');
GO
INSERT INTO [dbo].[data] ([id], [id_emp], [emp_make]) VALUES (N'5', null, N'无明');
GO

-- ----------------------------
-- Indexes structure for table data
-- ----------------------------

-- ----------------------------
-- Primary Key structure for table [dbo].[data]
-- ----------------------------
ALTER TABLE [dbo].[data] ADD PRIMARY KEY ([id])
GO

-- ----------------------------
-- Foreign Key structure for table [dbo].[data]
-- ----------------------------
ALTER TABLE [dbo].[data] ADD FOREIGN KEY ([id_emp]) REFERENCES [dbo].[employee] ([id]) ON DELETE NO ACTION ON UPDATE NO ACTION
GO

[解决办法]
UPDATE [data]
SET id_emp=employee.id
FROM [data] LEFT JOIN employee ON [data].emp_make=employee.emp
WHERE ISNULL([data].id_emp,'')=''
[解决办法]

 update [data]
 set [id_emp]=b.[id]
 from [data] a,[employee] b
 where (a.id_emp is null or a.id_emp='') and a.emp_make=b.emp

[解决办法]

 update [data]
 set [id_emp]=b.[id]
 from [employee] b
 where ([data].id_emp is null or [data].id_emp='') 
       and [data].emp_make=b.emp

[解决办法]

update a
 set a.id_emp=b.id
 from [data] a
 inner join [employee] b on b.emp=a.emp_make
 where a.id_emp is null or a.id_emp=''


select * from [data]

/*
id          id_emp      emp_make
----------- ----------- ----------
1           1           小明
2           2           小明
3           2           中明
4           3           大明
5           NULL        无明

(5 row(s) affected)
*/

[解决办法]

update [data]
set [id_emp]=b.[id]
from [employee] b
where ([data].id_emp is null or [data].id_emp='') 
   and [data].emp_make=b.emp
   
select *
from data
/*
idid_empemp_make
11小明
22小明
32小明
43大明
5NULL无明
*/