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Complex SQL joins can be a headache, especially when it comes to validating the results. When joining multiple tables, one may encounter unexpected results due to the complexity of join conditions or the presence of duplicate records. In this blog post, we will focus on the use of CHECKSUM and HASHBYTES to validate the results of complex joins. These two functions, available in SQL Server 2016 and 2017, as well as Azure SQL, can greatly simplify the process of data validation.

CHECKSUM is a function that provides a simple method for comparing data between tables. It computes a hash value for each row of data, which can then be compared to ensure data consistency. If the CHECKSUM values match, one can be confident that the data in the rows is identical. Here's an example of how you might use CHECKSUM in a SQL Server environment:

-- SQL Server example
SELECT CHECKSUM(*) FROM table1
INTERSECT
SELECT CHECKSUM(*) FROM table2

The above example will return rows that are identical between the two tables. However, CHECKSUM isn't perfect and on rare occasions, it may return the same value for different data. In such cases, HASHBYTES can be used as it provides a more robust method for comparing data.

HASHBYTES function in SQL Server generates a hash value in binary format for an input. It supports several algorithms, including MD5 and SHA1, which are more accurate in comparing data than CHECKSUM. Here's an example of how you might use HASHBYTES:

-- SQL Server example
SELECT HASHBYTES('SHA1', CONCAT(column1, column2, column3)) FROM table1
INTERSECT
SELECT HASHBYTES('SHA1', CONCAT(column1, column2, column3)) FROM table2

Unfortunately, MySQL 5.7 and DB2 11.1 do not support CHECKSUM and HASHBYTES directly. However, you can still use similar functions like MD5 to achieve the same result. Here's an example in MySQL:

-- MySQL 5.7 example
SELECT MD5(CONCAT(column1, column2, column3)) FROM table1
INTERSECT
SELECT MD5(CONCAT(column1, column2, column3)) FROM table2

In DB2, you can use HASHROW function for similar purposes:

-- DB2 11.1 example
SELECT HASHROW(column1, column2, column3) FROM table1
INTERSECT
SELECT HASHROW(column1, column2, column3) FROM table2

In conclusion, CHECKSUM and HASHBYTES provide powerful tools for validating complex joins in SQL Server and Azure SQL. While not directly available in MySQL and DB2, similar functions can be used to ensure data integrity. By incorporating these techniques into your SQL toolkit, you can more confidently handle complex joins and ensure accurate data.

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