How do I put an 'if clause' in an SQL string?

Question

So here\'s what I want to do on my MySQL database.

I would like to do:

SELECT *
    FROM itemsOrdered
    WHERE purchaseOrder_ID = \'@purchaseOr         


        

Answer 1

if not exists(select top 1 fromFROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status = 'PENDING' )
Begin

UPDATE purchaseOrder 
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID

End

Answer 2

You can use the multiple-table UPDATE syntax to effect an ANTI-JOIN between purchaseOrder and itemsOrdered:

UPDATE purchaseOrder p LEFT JOIN itemsOrdered i
    ON p.purchaseOrder_ID = i.purchaseOrder_ID
   AND i.status = 'PENDING'
SET    p.purchaseOrder_status = 'COMPLETED'
WHERE  p.purchaseOrder_ID = '@purchaseOrder_ID'
   AND i.purchaseOrder_ID IS NULL

Answer 3

You can also use the following query to check if the record exists and then update it:

if not exists(select top 1 fromFROM itemsOrdered
    WHERE purchaseOrder_ID = '@purchaseOrdered_ID'
        AND status = 'PENDING' )
Begin

UPDATE purchaseOrder 
    SET purchaseOrder_status = 'COMPLETED'
    WHERE purchaseOrder_ID = '@purchaseOrder_ID

End

Answer 4

If the table contains millions of records then the following query will work fast.

UPDATE PO
SET PO.purchaseOrder_status = 'COMPLETED'
FROM purchaseOrder PO
LEFT OUTER JOIN itemsOrdered IOD ON IOD.purchaseOrder_ID = PO.purchaseOrdered_ID and IOD.status = 'PENDING'
WHERE IOD.purchaseOrder_ID IS NULL

Answer 5

Since MySQL doesn't support if exists(*Your condition*) (*Write your query*), you can achieve an 'if clause' by writing like this:

(*Write your insert or update query*) where not exists (*Your condition*)

Answer 6

You can declare a variable holding the number of returned results on select query. You can then run the update statement if this variable is more than 0

    Declare @ResultCount int
    SELECT @ResultCount = count(*) FROM itemsOrdered WHERE purchaseOrder_ID = '@purchaseOrdered_ID' AND status = 'PENDING'        
    If @ResultCount > 0
UPDATE purchaseOrder SET purchaseOrder_status = 'COMPLETED' WHERE purchaseOrder_ID = '@purchaseOrder_ID'