Finding even or odd ID values

sql

prompt-engineering

best-practices

performance

byNikita Barsukov·Nov 25, 2024

Retrieve even IDs with ID % 2 = 0; for odd IDs, use ID % 2 = 1:

-- Only the chosen ones: Even IDs
SELECT * FROM table WHERE ID % 2 = 0;

-- The leftovers: Odd IDs
SELECT * FROM table WHERE ID % 2 = 1;

Understanding the magic: The modulo operator

The modulo operator % is essentially dividing and giving you the remainder after division. If you divide your ID by 2 and there's no remainder (i.e., the remainder is 0), then the ID is even.

Even IDs after modulo operation with 2 yields 0, because there's no remainder after division:

-- Even Stevens are always decisive, just like the modulo of an even number and 2
SELECT ID FROM table WHERE (ID % 2) = 0;

Odd IDs after modulo operation with 2 yields 1, because they do have a remainder after division:

-- Odd ones don't like fair divisions, they always have something to spare. That's why ID % 2 = 1
SELECT ID FROM table WHERE (ID % 2) = 1;

Working with different SQL dialects

When working on diverse platforms like DB2, MySQL, SQL Server, or Oracle, be aware of the syntax distinctions. In Oracle, you replace the % operator with the MOD function:

-- In Oracle land, Even folks still don't leave any leftovers 
SELECT * FROM table WHERE MOD(ID, 2) = 0;

-- But Odd folks, they still got something left in the fridge
SELECT * FROM table WHERE MOD(ID, 2) = 1;

This is a key awareness point when working with different database platforms.

Common pitfalls and how to avoid them

Nulls: If your ID column contains nulls, your SQL will throw a fit unless you handle it. Always perform a null check whenever you are not sure:

-- Null checks are like insurance. You hate dealing with them, but you'll wish you had one when things go south
SELECT * FROM table WHERE ID IS NOT NULL AND ID % 2 = 0;

Performance: For large datasets, indexes on the ID column can act as your GPS in a forest of data. Use them wisely for faster searches.
DataType Considerations: Ensure the ID field is numeric. SQL might start to behave weird and wild with the modulo operator and non-integer types.

Beyond basics: Advanced use cases

In the real world, things often get complex. Sometimes, you might want to classify IDs as even or odd by using a CASE WHEN statement:

-- Life is not always binary. But for our IDs, it pretty much is.
SELECT ID,
       CASE WHEN (ID % 2) = 0 THEN 'Even Steven'
            WHEN (ID % 2) = 1 THEN 'Odd Todd'
            ELSE 'The Ugly Duckling'
       END as IdentityClass
FROM table;