Need a row count after SELECT statement: what's the optimal SQL approach?

Question

I\'m trying to select a column from a single table (no joins) and I need the count of the number of rows, ideally before I begin retrieving the rows. I have come to two app

Answer 1

There are only two ways to be 100% certain that the COUNT(*) and the actual query will give consistent results:

• Combined the COUNT(*) with the query, as in your Approach 2. I recommend the form you show in your example, not the correlated subquery form shown in the comment from kogus.
• Use two queries, as in your Approach 1, after starting a transaction in SNAPSHOT or SERIALIZABLE isolation level.

Using one of those isolation levels is important because any other isolation level allows new rows created by other clients to become visible in your current transaction. Read the MSDN documentation on SET TRANSACTION ISOLATION for more details.

Answer 2

Why don't you put your results into a vector? That way you don't have to know the size before hand.

Answer 3

If you're using SQL Server, after your query you can select the @@RowCount function (or if your result set might have more than 2 billion rows use the RowCount_Big() function). This will return the number of rows selected by the previous statement or number of rows affected by an insert/update/delete statement.

SELECT my_table.my_col
  FROM my_table
 WHERE my_table.foo = 'bar'

SELECT @@Rowcount

Or if you want to row count included in the result sent similar to Approach #2, you can use the the OVER clause.

SELECT my_table.my_col,
    count(*) OVER(PARTITION BY my_table.foo) AS 'Count'
  FROM my_table
 WHERE my_table.foo = 'bar'

Using the OVER clause will have much better performance than using a subquery to get the row count. Using the @@RowCount will have the best performance because the there won't be any query cost for the select @@RowCount statement

Update in response to comment: The example I gave would give the # of rows in partition - defined in this case by "PARTITION BY my_table.foo". The value of the column in each row is the # of rows with the same value of my_table.foo. Since your example query had the clause "WHERE my_table.foo = 'bar'", all rows in the resultset will have the same value of my_table.foo and therefore the value in the column will be the same for all rows and equal (in this case) this the # of rows in the query.

Here is a better/simpler example of how to include a column in each row that is the total # of rows in the resultset. Simply remove the optional Partition By clause.

SELECT my_table.my_col, count(*) OVER() AS 'Count'
  FROM my_table
 WHERE my_table.foo = 'bar'

Answer 4

If you're concerned the number of rows that meet the condition may change in the few milliseconds since execution of the query and retrieval of results, you could/should execute the queries inside a transaction:

BEGIN TRAN bogus

SELECT COUNT( my_table.my_col ) AS row_count
FROM my_table
WHERE my_table.foo = 'bar'

SELECT my_table.my_col
FROM my_table
WHERE my_table.foo = 'bar'
ROLLBACK TRAN bogus

This would return the correct values, always.

Furthermore, if you're using SQL Server, you can use @@ROWCOUNT to get the number of rows affected by last statement, and redirect the output of real query to a temp table or table variable, so you can return everything altogether, and no need of a transaction:

DECLARE @dummy INT

SELECT my_table.my_col
INTO #temp_table
FROM my_table
WHERE my_table.foo = 'bar'

SET @dummy=@@ROWCOUNT
SELECT @dummy, * FROM #temp_table

Answer 5

If you are really concerned that your row count will change between the select count and the select statement, why not select your rows into a temp table first? That way, you know you will be in sync.

Answer 6

Approach 2 will always return a count that matches your result set.

I suggest you link the sub-query to your outer query though, to guarantee that the condition on your count matches the condition on the dataset.

SELECT 
  mt.my_row,
 (SELECT COUNT(mt2.my_row) FROM my_table mt2 WHERE mt2.foo = mt.foo) as cnt
FROM my_table mt
WHERE mt.foo = 'bar';