How do I compare two columns for equality in SQL Server?

Comparing columns from two different tables

Comparisons often involve different tables. Use a join operation in such cases:

-- Who knew SQL could play matchmaker?
SELECT a.*, b.*, 
       CASE WHEN a.column1 = b.column2 THEN 1 ELSE 0 END AS IsMatch
FROM Table1 a
INNER JOIN Table2 b ON a.PrimaryKey = b.ForeignKey;
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Two words: IIF function

SQL Server 2012 introduced the IIF function as a shorter alternative to CASE:

-- Because who doesn't like snack-sized code, right?
SELECT *, IIF(column1 = column2, 1, 0) AS IsMatch FROM your_table;
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Remember, backward compatibility issues may arise if you're using an older SQL Server version.

Null is not zero, friends!

While comparing, handle NULL values with utmost care:

-- Nulls are the ninjas of the SQL world. They are elusive!
SELECT ISNULL(NULLIF(column1, column2), 1) AS IsNotMatch FROM your_table;
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This efficiently returns 1 for non-matching columns, even considering NULLs.

Outer join: the detective of SQL

For robust comparisons encompassing all possible scenarios, apply a FULL OUTER JOIN:

-- Nothing escapes the vigilant eyes of an OUTER JOIN!
SELECT a.column1, b.column2, 
       CASE WHEN a.column1 = b.column2 THEN 'Equal' 
            WHEN a.column1 IS NULL OR b.column2 IS NULL THEN 'One Is Null'
            ELSE 'Not Equal' 
       END AS ComparisonResult
FROM Table1 a
FULL OUTER JOIN Table2 b ON a.PrimaryKey = b.ForeignKey;
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Hidden aspects of column comparison

Collations: the silent influencers

Collations dictate comparison rules - always check they match:

-- SQL version of 'When in Rome, do as the Romans do.'
SELECT *
FROM Table1
WHERE column1 COLLATE Latin1_General_CI_AS = column2 COLLATE Latin1_General_CI_AS;
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Pattern matching? Like!

Comparing string patterns? Here's how:

-- Wanted: column1. Last seen wearing: pattern of column2. 
SELECT *
FROM your_table
WHERE column1 LIKE column2; -- '%' and '_' are the Usain Bolts here!
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Note: Pattern matching may slow down your query!

Between me, you & the range

For numeric ranges, meet BETWEEN:

-- Play it safe, stay within the boundaries!
SELECT *
FROM your_table
WHERE column1 BETWEEN start_value AND end_value;
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Keep no secrets while comparing

Data types: apples & oranges

Comparing unlike data types might fail. Convert or cast as needed:

-- SQL: I can make an apple out of an orange. No big deal!
SELECT *
FROM your_table
WHERE CAST(column1 AS INT) = CAST(column2 AS INT);
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Queries: the fast and the furious

Comparisons can slow down your queries. Watch your indexing:

-- Someday, SQL will have its own version of 'Fast & Furious'
SELECT *
FROM your_table
WHERE indexed_column = non_indexed_column;
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Analyze your execution plans and fix indexing for speedy retrieval!