Difference between 2 indexes with columns defined in reverse order

Question

Are there any differences between following two indexes?

• IDX_IndexTables_1
• IDX_IndexTables_2

If there are any, what are the differences?

create table IndexTables (
    id int identity(1, 1) primary key,
    val1 nvarchar(100),
    val2 nvarchar(100),
)

create index IDX_IndexTables_1 on IndexTables (val1, val2)
GO

create index IDX_IndexTables_2 on IndexTables (val2, val1)
GO

Answer 1

Yes. There is a difference.

The composite index IDX_IndexTables_1 can be used for any query where the val1 column is used in the where clause.

The composite index IDX_IndexTables_2 can be used for any query where the val2 column is used in the where clause.

So, for instance IDX_IndexTables_2 cannot be used for this query (but IDX_IndexTables_1 can be used):

SELECT val1, val2 FROM IndexTables
WHERE val1 = some_value

but can be used for this query:

SELECT val1, val2 FROM IndexTables
WHERE val2 = some_value AND val1 = some_other-value

The way to think about a composite index is think about a paper telephone directory; It is indexed by the surname column, and then the firstname column: you can look up by surname but not by firstname on its own.