I have 2 tables with the following structure:

Table A:

```
id_A    col1
 1      val1
 2      val2
 3      val3
 ...    ....

```

Table B:

```
id_B  mycol   id_A_val
 1    smval1    null
 2     null      1
 3     null      2
...    ...       ...

```

I want to copy values from Table A's col1 into Table B's mycol
This is my expected result:

Expected:

```
id_B  mycol   id_A_val
 1    smval1    null
 2    val1      1
 3    val2      2
...    ...      ...

```

I tried several combinations of SQL UPDATE. This was the latest I tried - but it throws an error saying "Subquery returned more than 1 value."

Tried:

```
UPDATE [dbo].[Table_B] 
SET MYCOL = (SELECT inst.[COL1] FROM [dbo].[TABLE_A] a,  [dbo].[TABLE_B] b
WHERE a.[ID_A] = b.[ID_A_VAL] AND b.ID_A_VAL IS NOT NULL)

```

Can someone throw some light on the correct direction to get a working query?