Question

I tried to add a new column to a table in same db, I then tried to copy new column from another table’s column.

ALTER TABLE [library].[dbo].[Member$]
        add  new_column1 float
        go
        update [library] .[dbo] .[Member$] 
        set new_column1  = select [library].[dbo].Category$ .Category  from  [library] .[dbo] .[Category$]  
        GO
    CREATE TABLE [dbo].[Category$](
        [CatID] [float] NULL,
        [Category] [nvarchar](255) NULL,
        [BooksLimit] [float] NULL,
        [Period] [float] NULL,
        [FinePerDay] [float] NULL
    ) ON [PRIMARY]
    
    GO

CREATE TABLE [dbo].[Member$](
    [Roll No] [float] NULL,
    [RollNo] [nvarchar](255) NULL,
    [Name] [nvarchar](255) NULL,
    [CatID] [nvarchar](255) NULL,
    [ExpiryDate] [datetime] NULL,
    [ExpiryReason] [nvarchar](255) NULL,
    [Session] [nvarchar](255) NULL,
    [new_column] [float] NULL,
    [new_column1] [float] NULL
) ON [PRIMARY]

But the new_column1 is still null

There are multiple values in Category, but row number in both doesnot match, Is that a problem, If yes then how can I achieve this?

Answer 1

Try this:

ALTER TABLE [library].[dbo].[Member$] add new_column1 float 
go 
update [library] .[dbo] .[Member$] set 
  new_column1 = 
  (
    select TOP 1 [library].[dbo].[Category$].Category 
    from [library].[dbo].[Category$]
  )

BUT

Doing so you just assigns the same value to all the rows in table Member$, if you need specific settings for each Member$’s row – try to place proper filtering in the subquery

select TOP 1 [library].[dbo].[Category$].Category 
from [library].[dbo].[Category$]
WHERE PLACE_YOUR_FILTER_HERE

like this:

ALTER TABLE [library].[dbo].[Member$] add new_column1 float 
go 
update [library] .[dbo] .[Member$] set 
  new_column1 = 
  (
    select TOP 1 [library].[dbo].[Category$].Category 
    from [library].[dbo].[Category$]
    WHERE [library] .[dbo] .[Member$].CatId = [library].[dbo].[Category$].CatId
  )

BUT, special note

If you want to use ID for category – better use for it an exact type, such as int – because FLOAT types family are not exact and thus – harder to compare.