Question

I’m trying to avoid database access in loops within a project I am working on. Not being too good with SQL, I’m not sure of the best way to approach this.

I’m updating a stock level database in a sale procedure with multiple stock locations/pick locations.

Therefore, this is what I am doing.

Looping through Product IDs, then looping through the pick locations for each product and updating quantities as it goes, like:

For Each wProductId In calculatedProds.Keys '' loop through products requested passing values of pick locations
        For i = 0 To locationCount ' split the location value from the string as per above
            Dim thisLocation As Integer = locationID
            Dim thisQty As Integer = qtyPicked                
            Dim sql As String = "UPDATE `stockLevels` SET `stockLevel`=`stockLevel` - '" & thisQty & "' WHERE `stockLocation`='" & thisLocation & "' AND `id`='" & wProductId & "'"
            ' DO DATA ACCESS WITH SQL ABOVE       
        Next
Next

Of course this works, but it is opening a new Database Connection for every stock location, for every item.

So how would I work this into a single Update Statement?

http://www.karlrixon.co.uk/writing/update-multiple-rows-with-different-values-and-a-single-sql-query/

That link gets me very close to what I am after, I think, but I am not 100% sure how to dynamically build that SQL statement and how to add two conditions to the CASE.

I need to build a SQL Statement something like:

UPDATE stockLevels
SET stockLevel= CASE id
    WHEN '"& wProductId  &"' AND stockLocation='"& thisLocation &"' THEN `stockLevel` - '" & thisQty & "'
    WHEN '"& NEXTwProductId  &"' AND stockLocation='"& NEXTthisLocation &"' THEN `stockLevel` - '" & NEXTthisQty & "'
END

But that’s not correct where I am adding the second parameter to the CASE!

I am using MySQL and VB.NET, as usual, any help much appreciated.

Answer 1

Your CASE expression is simply incorrect syntactically.

There are two kinds of CASE expressions, almost identical to each other and yet slightly different in syntax.

One has the form of

CASE expr
  WHEN value1 THEN result1
  WHEN value2 THEN result2
  ...
  ELSE result_else
END

The other looks like this:

CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ...
  ELSE result_else
END

And you were essentially attempting to mix these two kinds of CASE.

You probably just need to use the second one (also called search CASE, if I am not much mistaken):

...
CASE
  WHEN id = '"& wProductId  &"' AND stockLocation='"& thisLocation &"' THEN ...
  WHEN id = '"& NEXTwProductId  &"' AND stockLocation='"& NEXTthisLocation &"' THEN ...
...

Note that if there’s no match and the CASE has no ELSE part, the result will be NULL, so make sure you’ve covered all the cases, otherwise use an ELSE part like this:

ELSE `stocklevel`

I.e. the CASE will evaluate to the original value of the column being updated, rendering no update for it in the end.