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How to determine of if columns exist in one table and not another and alter the table

Posted on 2010-11-24
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Last Modified: 2015-02-09
I have two tables of which one is a backup table.  I need to dynamically add columns to the backup table if they existi in the primary table but not in the backup table.
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Question by:ccunni1
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6 Comments
 
LVL 60

Expert Comment

by:Kevin Cross
ID: 34208259
Hi-

You can use a SQL statement like below against the INFORMATION_SCHEMA to build dynamic ALTER TABLE statements.

The query will return for you the columns and associated information that are in the live table that do not exist with same name in the backup table.

You can then use columns like COLUMN_NAME, COLUMN_DEFAULT, IS_NULLABLE, DATA_TYPE, et cetera to build your ALTER TABLE {backup table name} ADD ... statements.
http://msdn.microsoft.com/en-us/library/aa275462(SQL.80).aspx

Hope that helps!

Kevin
select *
from information_schema.columns c1
where c1.TABLE_NAME = '{live table name}'
and not exists (
   select null
   from information_schema.columns c2
   where c2.TABLE_NAME = '{backup table name}'
   and c2.COLUMN_NAME = c1.COLUMN_NAME
);

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LVL 58

Expert Comment

by:cyberkiwi
ID: 34208263
Say your tables are t_1 and t_2
declare @sql varchar(8000)
set @sql = ''
select @sql = @sql + 'alter table t_2 add ' + quotename(c.name) + ' ' + t.name +
case
when t.name in (
	'varbinary','varchar','binary','char','nchar','float',
	'time','datetime2','datetimeoffset')
	then '(' + right(c.xprec,10) + ')'
when t.name = 'nvarchar'
	then '(' + right(c.length/2,10) + ')'
when t.name in ('decimal','numeric')
	then '(' + right(c.xprec,10) + ',' + right(c.xscale,10) + ')'
else
	''
end
+ ';'
from 
(
select a.*
from
(select c.* from sysobjects o inner join syscolumns c on c.id=o.id where o.name = 't_1') a
left join
(select c.* from sysobjects o inner join syscolumns c on c.id=o.id where o.name = 't_2') b
on a.name=b.name
where b.name is null
) c
inner join systypes t on t.xtype=c.xtype and t.xusertype = c.xusertype
;
--print @sql
exec (@sql)

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Author Comment

by:ccunni1
ID: 34232792
I am getting a Msg 1001, Level 15, State 1, Line 1
Line 1: Length or precision specification 0 is invalid.
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LVL 58

Accepted Solution

by:
cyberkiwi earned 500 total points
ID: 34232955
I added a condition for varchar/varbinary max..

Regards
declare @sql varchar(8000)
set @sql = ''
select @sql = @sql + 'alter table t_2 add ' + quotename(c.name) + ' ' + t.name +
case
when t.name in ('varbinary','varchar') AND c.xprec=0 then
	'(max)'
when t.name in (
	'varbinary','varchar','binary','char','nchar','float',
	'time','datetime2','datetimeoffset')
	then '(' + right(c.xprec,10) + ')'
when t.name = 'nvarchar'
	then '(' + right(c.length/2,10) + ')'
when t.name in ('decimal','numeric')
	then '(' + right(c.xprec,10) + ',' + right(c.xscale,10) + ')'
else
	''
end
+ ';'
from 
(
select a.*
from
(select c.* from sysobjects o inner join syscolumns c on c.id=o.id where o.name = 't_1') a
left join
(select c.* from sysobjects o inner join syscolumns c on c.id=o.id where o.name = 't_2') b
on a.name=b.name
where b.name is null
) c
inner join systypes t on t.xtype=c.xtype and t.xusertype = c.xusertype
;
--print @sql
exec (@sql)

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LVL 22

Expert Comment

by:Steve Wales
ID: 40597946
This question has been classified as abandoned and is closed as part of the Cleanup Program. See the recommendation for more details.
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