dbForge Data Compare for SQL Server
A powerful, fast, and easy-to-use data comparison tool
How to Compare Rows and Columns in SQL Server Databases
Whether you are a developer, database administrator, or architect, understanding how to effectively compare data in SQL Server databases is an essential skill. Such data comparisons provide you with insights into database behavior and help you validate and troubleshoot database structure and data.
In this article, we will cover various methods and techniques for comparing rows and columns using SQL queries and dbForge Data Compare for SQL Server.
Why you need to compare rows or columns in SQL
Comparing rows and columns in SQL Server databases helps you ensure data integrity and quickly pinpoint differences that may affect your application performance. With data at the core of multiple workflows, it is critical for it to remain consistent and reliable throughout its lifecycle and continue serving as the source of truth for people in various roles. By comparing columns and rows, you can find differences to identify data changes or inconsistencies as part of different workflows:
- Keeping data in sync across multiple databases
- Identifying and correcting data discrepancies after a migration, replication, or backup
- Validating schema change or migration results
- Detecting unexpected changes that may indicate security issues
- Tracking data changes for compliance purposes
- Verifying application logic correctness by comparing data between environments
Thus, understanding how to compare columns and/or rows in SQL can help you maintain data consistency and database integrity.
How to generate SQL examples for comparing rows or columns
As a prerequisite, we have created SQL tables and populated them with test data for comparing rows and columns in SQL Server. This can usually be done manually with the help of the CREATE and INSERT statements. However, you can simplify this process and use dbForge Data Generator for SQL Server. The tool allows you to quickly populate tables with dummy data in SQL Server.
What are comparison operators in SQL Server?
SQL comparison operators can be used in condition clauses to compare an expression to another value or expression. These operators compare the value stored in a column to a specified value and return only those values that meet the specified condition.
| Operator | Description |
|---|---|
| = | Equal to |
| != or <> | Not equal to |
| > | Greater than |
| >= | Greater than or equal to |
| < | Less than |
| <= | Less than or equal to |
How to compare data in SQL Server
SQL supports multiple operations and functions that allow you to compare data between rows and columns within the same table or multiple tables. When used within the intuitive GUI of dbForge Data Compare for SQL Server, these operations and functions provide you with deeper insights into your data.
Below is a list of the most commonly used SQL comparison techniques.
| Comparison task | SQL technique / tool |
|---|---|
| Compare two rows in the same table | Use SELF JOIN with aliases (for example, FROM table t1 JOIN table t2 ON t1.colA = t2.colB) |
| Compare two columns across different tables | Use WHERE, JOIN, or UNION to match values or detect mismatches between tables |
| Compare multiple columns | Wrap comparisons in a CASE statement to flag statuses (for example, Active/Inactive) |
| Compare two values in the same row | Use direct comparison operators in WHERE clause, for example, WHERE col1 <> col2 |
| Compare two rows of the same column | Use window functions like LAG() to compare current vs. previous row values |
| Find date differences | Use built-in date functions or a tool like dbForge Data Compare to highlight discrepancies |
| Perform full table or table-to-table comparison | Use LEFT JOIN and set-based operators: UNION ALL, INTERSECT, EXCEPT |
Read on for detailed explanations and examples of each technique.
How to compare two columns in SQL Server
SQL allows for the comparison of columns either within the same table or across different tables.
Compare two columns within the same table
A SELF JOIN, which joins a table to itself, can be used to compare columns within the same table. The syntax for SELF JOIN would be:
SELECT column_names FROM table1 t1, table1 t2 WHERE condition_a AND condition_b;
The pattern includes the table name - table1 - joined to itself with different t1 and t2 aliases, and the join condition (condition_a and condition_b) specifying the columns to be compared. The use of aliases allows for renaming the columns in a table so the database engine can execute your query.
Note
You need to use aliases because the columns you join have the same column names.
In the following example, we are going to compare different product models and find those that have the same list price. The query and output will be as follows:
Compare two columns from different tables
When comparing two columns between different tables, you can use the following techniques:
- WHERE clause, which compares and retrieves data based on the specified condition.
- JOIN, which joins two tables on common columns to compare and retrieve data from both tables.
- UNION operator, which combines the results of two SELECT statements, removes duplicates, and compares the data in the resulting table. Both statements must have the same number of columns, similar data types, and the same order of columns. Unlike JOIN and WHERE, the UNION operator processes the NULL values.
An example of using the UNION operator is to compare data in two tables - Department and EmployeeDepartmentHistory - by combining the results of SELECT statements on each table. In this example, we request records where DepartmentID = 1 and ShiftID = 1 and obtain the results in a single grid.
Note
To align table structures, use NULL for columns that do not exist in both tables.
How to compare values in multiple columns in SQL Server
To compare values stored in several SQL columns, you can use the CASE statement, which returns a result based on the comparison. The CASE statement includes at least one pair of WHEN and THEN statements. The WHEN clause defines the condition to be checked. The THEN clause specifies the action if the WHEN condition returns TRUE.
In the following example, we will use the SQL CASE statement to set the value of the status column to 'Normal' if the Quantity column has a value greater than 200; otherwise, the value for the status column will be set to 'Low'.
How to compare rows in the same table
You can compare values in the same row or across two rows of the same column.
Compare two values in the same row
To compare two values of the same row, you can simply use the comparison operator in the WHERE clause. For example, if you want to compare a standard cost and a list price of products, execute the following statement:
SELECT Name ,ProductID ,ListPrice ,StandardCost FROM Production.Product WHERE StandardCost < ListPrice ORDER BY ListPrice;
Compare two rows of the same column
In SQL, the LAG() function is used to retrieve a value from a previous row in a result set. You can use the LAG() function to compare two rows of the same column by retrieving the value of a column from the previous row and comparing it to the current row.
The syntax of the LAG() function would be:
LAG(return_value ,offset [,default])
OVER (
[PARTITION BY partition_expression ]
ORDER BY sort_expression [ASC | DESC]
);
In this syntax:
- return_value is the value from the previous row, which will be returned based on a given offset.
- offset is the number of previous rows to retrieve data from, relative to the current row. If not specified, the default value is 1. It is an optional argument.
- default is the value to be returned if the offset exceeds the scope of the partition.
- PARTITION BY splits the result set into groups returned from the FROM clause. If not specified, the result set will be considered as a single group.
- ORDER BY sorts the result set by a specified sort expression.
Here is an example of the LAG function - the SELECT statement uses the LAG() function to compare the YTD sales between business entities. The function orders the list by business entity and compares the sales amount with the amount in the previous row:
SELECT BusinessEntityID ,SalesYTD ,LAG(SalesYTD) OVER ( ORDER BY BusinessEntityID ) previous_sales FROM Sales.SalesPerson;
In the output, the first row is NULL because it does not have any previous rows. The second row contains the value from the previous row due to the default offset value 1.
How to find the difference between date values in SQL
In SQL Server databases, dates can be stored with different date data types. However, for comparison, the date value specified by a user should match the value stored in a column; otherwise, the comparison fails. The easiest way to find the difference in date values is to use dbForge Data Compare for SQL Server. The tool allows for comparing date values having different date formats as part of a comprehensive database comparison.
To check for differences in date values, run a database comparison and navigate to the comparison results grid. Double-click a date value, and the differences will be highlighted in the Data Viewer window.
Conclusion
In SQL, you can compare rows and columns using different techniques, such as SELF JOINs, comparison operators, subqueries, UNION operator, LAG() function, or even specialized data comparison tools. Of course, any of these techniques may have their benefits or limitations. Which technique or tool to choose will depend on your task at hand. However, as you may have noticed, dbForge Data Compare can definitely ensure more accurate and consistent data comparison and synchronization. Besides, the tool allows you to review and resolve any data discrepancies as well as automate the synchronization process.
FAQ
How can I compare two columns in the same SQL Server table?
To compare columns in the same table, use a SELF JOIN operator joining a table to itself and assign aliases to each table copy to ensure different column names. Then execute a SELECT command specifying the columns you need to compare.
What is the best way to find differences between two tables in SQL Server?
To get a complete view of the differences between two tables in SQL Server, run a database comparison in dbForge Data Compare for SQL Server. The comparison results will include all the differences in data between the two databases.
Can I compare rows in SQL Server without writing complex queries?
To compare rows of the same column in a table, use the LAG() function that retrieves values from the previous row. The basic syntax of the LAG() function is as follows:
LAG(return_value ,offset [,default])
OVER (
[PARTITION BY partition_expression ]
ORDER BY sort_expression [ASC | DESC]
);
In this syntax:
- return_value is the value from the previous row, which will be returned based on a given offset.
- offset is the number of previous rows to retrieve data from, relative to the current row. If not specified, the default value is 1. It is an optional argument.
- default is the value to be returned if the offset exceeds the scope of the partition.
- PARTITION BY splits the result set into groups returned from the FROM clause. If not specified, the result set will be considered as a single group.
- ORDER BY sorts the result set by a specified sort expression.
How do I highlight mismatched data between two SQL Server databases?
The fastest and easiest way to examine mismatched data between two SQL Server databases is to use dbForge Data Compare. First, run a comparison of any two required databases. Then, in the comparison results window, double-click the data you need to view, which opens the Data Viewer window. It will display the data in both databases with differences highlighted.
Is there a GUI tool to automate row and column comparisons in SQL Server?
dbForge Data Compare for SQL Server provides a user-friendly GUI for creating and executing comparison queries that can be used in automation flows.
What SQL functions help detect changes across table versions?
To find changes between table versions, use set-based operators: UNION ALL, INTERSECT, EXCEPT.
How do I compare values in the same row across different columns?
To compare values in the same row, create a SELECT statement with the corresponding comparison operator (=, >, <, etc.) to retrieve values that match the condition.
Can I filter specific columns or rows when comparing SQL data?
To compare certain columns, specify them in the SELECT statement. Instead of SELECT *, explicitly state the names of columns you want to compare: SELECT [columnName]. To filter rows, use the WHERE clause limiting the results to rows that match the condition.
How do I track historical changes in a column using SQL?
To track historical changes, create a trigger that is activated whenever a value is updated and specify where the updated values need to be inserted.
How can we compare two columns in SQL?
You can compare columns either within the same table or between two tables. To compare columns within the same table, use a SELF JOIN operation to join a table to itself and create a SELECT statement specifying the columns to be compared.
To compare columns between two tables, use a UNION operator that combines results of two select statements in a single grid.