How to create fast database queries

Archive for February 10th, 2010


with 2 comments

Answering questions asked on the site.

myst asks:

I have two really large tables with lots of columns, many of them are nullable

I need to remove all rows from table1 which are not present in table2, but there can be duplicates in both tables and writing more than 70 IS NULL conditions would be a pain, and I want to make sure there's nothing I'm missing.

Is there a more simple way?

SQL Server supports EXCEPT clause which returns all records present in the first table and absent in the second one. But this clause eliminates duplicates and cannot be used as a subject to a DML operation.

ANSI SQL standard describes EXCEPT ALL which returns all records from the first table which are not present in the second table, leaving the duplicates as is. Unfortunately, SQL Server does not support this operator.

Similar behavior can be achieved using NOT IN or NOT EXISTS constructs. But in SQL Server, IN predicate does not accept more than one field. NOT EXISTS accepts any number of correlated columns, but it requires extra checks in the WHERE clause, since equality operator does not treat two NULL values as equal. Each pair or nullable columns should be additionally checked for a NULL in both fields. This can only be done using OR predicates or COALESCE, neither of which adds to performance.

But there is a way to emulate EXCEPT ALL in SQL Server quite elegantly and efficiently.

Let's create two sample tables:
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Written by Quassnoi

February 10th, 2010 at 11:00 pm

Posted in SQL Server