问题描述

我需要一个 SQL 语句，用 #T2(C1) 中的值填充 #T1 表第二列中的空值.

I need a SQL statement which fills the null values from the second column of #T1 table with values from #T2(C1).

这两个表的列之间没有外键或匹配项.

There is no foreign key or match between the columns of those two tables.

示例:

T1 (C1, T2C1)
A1, 1
A2, null
A3, null
A4, 4
A5, null
-------------
T2 (C1)
a
b

更新后，T1 将如下所示:

After update, the T1 will look like:

A1, 1
A2, a
A3, b
A4, 4
A5, null

我找到了两种方法:

使用 CTE

create table #T1 (C1 varchar(10), T2C1 varchar(10))
create table #T2 (C1 varchar(10))

insert into #T1 values ('A1', '1')
insert into #T1 values ('A2', null)
insert into #T1 values ('A3', null)
insert into #T1 values ('A4', '4')
insert into #T1 values ('A5', null)

insert into #T2 values ('a')
insert into #T2 values ('b')

;with t2 as
(
select C1, row_number() over (order by C1) as Index2
from #T2
)
,t1 as
(
select T2C1, row_number() over (order by C1) as Index1   
from #T1
where T2C1 is null
)
update t1
set t1.T2C1 = t2.C1
from t2
where t1.Index1 = t2.Index2

select * from #T1

drop table #T1
drop table #T2

带有派生表

create table #T1 (C1 varchar(10), T2C1 varchar(10))
create table #T2 (C1 varchar(10))

insert into #T1 values ('A1', '1')
insert into #T1 values ('A2', null)
insert into #T1 values ('A3', null)
insert into #T1 values ('A4', '4')
insert into #T1 values ('A5', null)

insert into #T2 values ('a')
insert into #T2 values ('b')

update #T1
set T2C1 = cj.C1
from #T1
join (select T2C1, row_number() over (order by C1) as Index1, C1
   from #T1
   where T2C1 is null) ci on ci.C1 = #T1.C1
join (select C1, row_number() over (order by C1) as Index2
  from #T2) cj on ci.Index1 = cj.Index2

select * from #T1

drop table #T1
drop table #T2

我的问题是，我可以在不使用窗口函数且不使用光标的情况下实现这一点吗?

My question is, can I achieve this without using windowing functions and with no cursors?

更新
@Damien_The_Unbeliever 正确地指出要进行这种更新，如果不定义表的排序是不可能的，实际上我认为确切地说是没有正确识别和链接目标表中的行.
@Bogdan Sahlean 找到了另一种方法，使用表变量和 IDENTITY 列，我对这个解决方案很满意，这是另一种方法但是，在实际应用中我仍然会使用窗口函数
谢谢大家

Update
@Damien_The_Unbeliever correctly points that to do this kind of update it is not possible without defining an ordering on tables, actually I think exactly said is without properly identify and link the rows from target table.
@Bogdan Sahlean has found another way, using table variables and IDENTITY column, which I'm happy with this solution, it's another way However, in the real application I will still use the windowing functions
Thanks all

推荐答案

1.我想你在目标表 (#T1) 中有一个 pk.

1.I suppose you have a pk in target table (#T1).

2.该解决方案使用 IDENTITY(1,1) 列和两个表变量代替 ROW_NUMBER.

2.Instead of ROW_NUMBER this solution uses IDENTITY(1,1) columns and two table variables.

3.我没有测试过这个解决方案.

3.I didn't tested this solution.

DECLARE @t2_count INT = (SELECT COUNT(*) FROM #T2);

DECLARE @Target TABLE
(
     MyId INT IDENTITY(1,1) PRIMARY KEY
    ,T1_pk INT NOT NULL UNIQUE
);
INSERT  @Target (T1_pk)
SELECT  TOP(@t2_count) pk
FROM    #T1 
WHERE   T2C1 IS NULL;

DECLARE @Source TABLE
(
     MyId INT IDENTITY(1,1) PRIMARY KEY
    ,C1 VARCHAR(10) NOT NULL
);
INSERT  @Source (C1)
SELECT  C1
FROM    #T2;

UPDATE  #T1
SET     T2C1 = src.C1
FROM    #T1 t
INNER JOIN @Target trg ON t.pk = trg.T1_pk 
INNER JOIN @Source src ON trg.MyId = src.MyId;

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