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Wednesday, 5 September 2012

The shortest, fastest, and easiest way to compare two tables in SQL Server

When you have two tables (or resultsets from SELECT statements) that you wish to compare, and you want to see any changes in ANY columns, as well as to see which rows exist in 1 table but not the other (in either direction) I have found that the UNION operator works quite well.
UNION allows you to compare all columns very quickly, and also handles comparing NULL values to other NULLs successfully, which a join clause or a WHERE condition doesn't normally do.  It also allows you to very quickly see which rows are missing in either table, which only a FULL OUTER JOIN will do, but of course we all know to avoid those at all costs (right?) -- a full outer join is about as “unrelational” as you can get.  (every column returned is potentially Null and must be wrapped in a COALESCE function).  Best of all, the UNION is quick and easy and short.
The basic idea is: if we GROUP the union of two tables on all columns, then if the two tables are identical all groups will result in a COUNT(*) of 2.  But for any rows that are not completely matched on any column in the GROUP BY clause, the COUNT(*) will be 1 -- and those are the ones we want.  We also need to add a column to each part of the UNION to indicate which table each row comes from, otherwise there is no way to distinguish between which row comes from which table.

CREATE PROCEDURE CompareTables(@table1 varchar(100),
 @table2 Varchar(100), @T1ColumnList varchar(1000),
 @T2ColumnList varchar(1000) = '')
declare @SQL varchar(8000);

IF @t2ColumnList = '' SET @T2ColumnList = @T1ColumnList

set @SQL = 'SELECT ''' + @table1 + ''' AS TableName, ' + @t1ColumnList +
 ' FROM ' + @Table1 + ' UNION ALL SELECT ''' + @table2 + ''' As TableName, ' +
 @t2ColumnList + ' FROM ' + @Table2

set @SQL = 'SELECT Max(TableName) as TableName, ' + @t1ColumnList +
 ' FROM (' + @SQL + ') A GROUP BY ' + @t1ColumnList +
 ' HAVING COUNT(*) = 1'

exec ( @SQL)

Parameter definition:

-- Table1, Table2 are the tables or views to compare.
-- T1ColumnList is the list of columns to compare, from table1.
-- Just list them comma-separated, like in a GROUP BY clause.
-- If T2ColumnList is not specified, it is assumed to be the same
-- as T1ColumnList.  Otherwise, list the columns of Table2 in
-- the same order as the columns in table1 that you wish to compare.
-- The result is all rows from either table that do NOT match
-- the other table in all columns specified, along with which table that row is from.

Happy learning......