Thursday, October 15, 2009

SQL Lookups

I am working with an Access app that was converted to use a SQL Server database, and gradually replacing Access queries with T-SQL or stored procedures for better performance.

If you have used Access for a while, you know that the so-called Domain Aggregate functions such as DCount, DLookup, and DSum are very expensive in terms of resource usage. Even if you write your own VBA function to perform the lookup, the fact is that the Jet database engine is not efficient when looking up a single value.

My challenge was that my form needed to pull data from 3 different queries. However, after binding a query to the form to get the 1st value, using DCount for the 2nd value & then DLookup for the 3rd, the form would open quite slowly. In fact, you could observe the various controls on the form being painted as the 3 values were pulled in.

The solution was to create a stored procedure to look up the values, create a pass-thru query to execute the sp, and then open a DAO recordset in the Form_Open event & grab the values from the pass-thru query.

Each of the Access queries / lookups were re-written using T-SQL. Then we declare 3 variables, use the SQL queries to get the values, and write a SELECT statement to return the values; something like this:

@value_1 int,
@value_2 int,
@value_3 int

SELECT @value_1 = (first query converted to T-SQL)

SELECT @value_2 = (second query converted to T-SQL)

SELECT @value_3 = (third query converted to T-SQL)

@value_1 AS value_1,
@value_2 AS value_2,
@value_3 AS value_3

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