Help with Writing a Query to Compare Two Tables in SQL Server

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Ravi Vishwakarma · Answer

To find records that are present in the OldCustomers table but not in the
NewCustomers table, you can use a LEFT JOIN combined with a
WHERE clause to identify records in OldCustomers that do not have corresponding records in
NewCustomers.
Here’s a general query for this scenario:
SELECT o.*
FROM OldCustomers o
LEFT JOIN NewCustomers n
    ON o.CustomerID = n.CustomerID
WHERE n.CustomerID IS NULL;
In this query:
o is an alias for OldCustomers.n is an alias for NewCustomers.The LEFT JOIN ensures that all records from OldCustomers are included, even if there is no matching record in
NewCustomers.The WHERE n.CustomerID IS NULL clause filters out records that have matches in
NewCustomers, leaving only those that do not.
If your tables have multiple columns that need to be checked for matching (e.g.,
CustomerID, Name, Email), you can adjust the
ON clause accordingly:
SELECT o.*
FROM OldCustomers o
LEFT JOIN NewCustomers n
    ON o.CustomerID = n.CustomerID
   AND o.Name = n.Name
   AND o.Email = n.Email
WHERE n.CustomerID IS NULL;
This ensures that a record is considered a match only if all specified columns match. Adjust the column names as necessary based on the structure of your tables.
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Help with Writing a Query to Compare Two Tables in SQL Server

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