The @tableName syntax is a table variable. They are rather limited. The syntax is described in the documentation for DECLARE @local_variable. You can kind of have indexes on table variables, but only indirectly by specifying PRIMARY KEY and UNIQUE constraints on columns. So, if your data in the columns that you need an index on happens to be unique, you can do this. See this answer. This may be “enough” for many use cases, but only for small numbers of rows. If you don’t have indexes on your table variable, the optimizer will generally treat table variables as if they contain one row (regardless of how many rows there actually are) which can result in terrible query plans if you have hundreds or thousands of rows in them instead.
The #tableName syntax is a locally-scoped temporary table. You can create these either using SELECT…INTO #tableName or CREATE TABLE #tableName syntax. The scope of these tables is a little bit more complex than that of variables. If you have CREATE TABLE #tableName in a stored procedure, all references to #tableName in that stored procedure will refer to that table. If you simply reference #tableName in the stored procedure (without creating it), it will look into the caller’s scope. So you can create #tableName in one procedure, call another procedure, and in that other procedure read/update #tableName. However, once the procedure that created #tableName runs to completion, that table will be automatically unreferenced and cleaned up by SQL Server. So, there is no reason to manually clean up these tables unless if you have a procedure which is meant to loop/run indefinitely or for long periods of time.
You can define complex indexes on temporary tables, just as if they are permanent tables, for the most part. So if you need to index columns but have duplicate values which prevents you from using UNIQUE, this is the way to go. You do not even have to worry about name collisions on indexes. If you run something like CREATE INDEX my_index ON #tableName(MyColumn) in multiple sessions which have each created their own table called #tableName, SQL Server will do some magic so that the reuse of the global-looking identifier my_index does not explode.
Additionally, temporary tables will automatically build statistics, etc., like normal tables. The query optimizer will recognize that temporary tables can have more than just 1 row in them, which can in itself result in great performance gains over table variables. Of course, this also is a tiny amount of overhead. Though this overhead is likely worth it and not noticeable if your query’s runtime is longer than one second.
for example, you can create the PRIMARY KEY on a temp table.
IF OBJECT_ID('tempdb..#tempTable') IS NOT NULL
DROP TABLE #tempTable
CREATE TABLE #tempTable
(
Id INT PRIMARY KEY
,Value NVARCHAR(128)
)
INSERT INTO #tempTable
VALUES
(1, 'first value')
,(3, 'second value')
-- will cause Violation of PRIMARY KEY constraint 'PK__#tempTab__3214EC071AE8C88D'. Cannot insert duplicate key in object 'dbo.#tempTable'. The duplicate key value is (1).
--,(1, 'first value one more time')
SELECT * FROM #tempTableReference
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