Tables or views that could not be compared

Objects with rows that can not be matched automatically are shown in the Tables or views that could not be compared group. To match rows in the two data sources, SQL Data Compare requires:

a comparison key for each table or view
mappings between objects in the data sources

In some cases, SQL Data Compare is unable to map objects and select comparison keys automatically. For example, if there are substantial schema differences between the data sources. This article provides information on manually setting comparison keys.

For more detailed information on setting mappings, see:

What is a comparison key?

When comparing data sources, SQL Data Compare looks for a matching primary key or other unique identifier in each data source to use as the comparison key. This enables matching rows to be identified, and their differences to be compared.

For example, the databases WidgetSales and WidgetDeploy each contain the table [dbo].[WidgetPrices]:

SDC_article_WidgetExample

Since rows can be inserted and deleted, we can not be certain that the third row in WidgetSales is the same as the third row in WidgetDeploy. Simply comparing rows in the order in which they appear in the table can result in a meaningless comparison. Similarly, rows can not be matched based on their Price values - more than one widget can have the same price.

In [dbo].[WidgetPrices], RecordID is a primary key. No two widgets will have the same RecordID, and the rows are uniquely identified. Two matching RecordID values in WidgetSales and WidgetDeploy therefore represent the same piece of real world data.

Where there is no matching primary key, or other unique identifier you must set an appropriate comparison key when selecting tables and views for comparison.

For example, if you know that two widgets with the same name are never added to the database on the same day, you can select the two columns WidgetName and DateValidFrom to form the comparison key. The rows in the tables can now be matched.

Setting comparison keys

SQL Data Compare automatically selects a comparison key when:

tables contain a matching primary key, unique index, or unique constraint
views contain a matching unique clustered index

If SQL Data Compare is unable to identify a suitable comparison key for a table or view, Not Set is shown in the Comparison Key box.

To set the comparison key for an object, click its Comparison Key box. A dialog box is displayed on which you can select the columns that will comprise the key:

Note that:

A comparison key cannot include columns whose data type is image, ntext, nvarchar(max), sql_variant, text, varbinary(max), varchar(max), or xml.
You cannot specify custom comparison keys if you are using a backup as a data source; however, you can select an alternative unique index or unique constraint.

For large databases, specifying a clustered index as the comparison key can result in improved performance.

For more information on setting up your comparison, see:

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