how to optimized sql query for speed up performance - sql

Question

i just have one sub query that just retrieves thousands of rows. here is my query :
str = "select * from ";
str += " (SELECT a.[ID] ,a.[strUserName], ";
str += " (select intCurrentBalance from accountmappings where intUserID = a.[ID] and intAccountType = 1) as Prm_Balance ";
str += " ,(select intCurrentBalance from accountmappings where intUserID = a.[ID] and intAccountType = 2 ) as Trn_Balance ";
str += " ,(select intCurrentBalance from accountmappings where intUserID = a.[ID] and intAccountType = 3 ) as Opt_Balance ";
str += " ,a.[strMobile] ";
str += " ,a.[strEmailID] ";
str += " ,a.[bIsApproved] ";
str += " ,a.[bIsActive] ";
str += " ,a.[dtlastrecharge] ";
str += " ,case when b.id is null then 0 else b.id end as createdbyId ";
str += " ,case when b.strusername is null then '' else b.strusername end as createdby ";
str += " FROM users a left join users b on a.intCreatedBy = b.ID ";
str += " ) as test ";
str += " where Prm_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Trn_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Opt_Balance < " + int.Parse(txt_balance.Text.Trim());
but this query takes too much time to display data. what resolution for this.

Answer 1

You can use single join for multiple account type and use CASE in the column selection as below
str = "select * from ";
str += " (SELECT a.[ID] ,a.[strUserName], ";
str += " CASE WHEN am.intAccountType = 1 THEN am.intCurrentBalance ELSE 0 END as Prm_Balance ";
str += " ,CASE WHEN am.intAccountType = 2 THEN am.intCurrentBalance ELSE 0 END as Trn_Balance ";
str += " ,CASE WHEN am.intAccountType = 3 THEN am.intCurrentBalance ELSE 0 END as Opt_Balance ";
str += " ,a.[strMobile] ";
str += " ,a.[strEmailID] ";
str += " ,a.[bIsApproved] ";
str += " ,a.[bIsActive] ";
str += " ,a.[dtlastrecharge] ";
str += " ,case when b.id is null then 0 else b.id end as createdbyId ";
str += " ,case when b.strusername is null then '' else b.strusername end as createdby ";
str += " FROM users a left join users b on a.intCreatedBy = b.ID ";
str += " left join accountmappings am ON am.intUserID = a.id
AND am.intAccountType in (1,2,3) ";
str += " ) as test ";
str += " where Prm_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Trn_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Opt_Balance < " + int.Parse(txt_balance.Text.Trim());

Answer 2

One obvious improvement is to remove subqueries and use them in the left joins as follows:
str = "select * from ";
str += " (SELECT a.[ID] ,a.[strUserName], ";
str += " a1.intCurrentBalance as Prm_Balance ";
str += " ,a2.intCurrentBalance as Trn_Balance ";
str += " ,a3.intCurrentBalance as Opt_Balance ";
str += " ,a.[strMobile] ";
str += " ,a.[strEmailID] ";
str += " ,a.[bIsApproved] ";
str += " ,a.[bIsActive] ";
str += " ,a.[dtlastrecharge] ";
str += " ,case when b.id is null then 0 else b.id end as createdbyId ";
str += " ,case when b.strusername is null then '' else b.strusername end as createdby ";
str += " FROM users a left join users b on a.intCreatedBy = b.ID ";
str += " left join accountmappings a1 on (a1.intUserID = a.[ID] and a1.intAccountType = 1) ";
str += " left join accountmappings a2 on (a2.intUserID = a.[ID] and a2.intAccountType = 2) ";
str += " left join accountmappings a3 on (a3.intUserID = a.[ID] and a3.intAccountType = 3) ";
str += " ) as test ";
str += " where Prm_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Trn_Balance < " + int.Parse(txt_balance.Text.Trim()) + " and Opt_Balance < " + int.Parse(txt_balance.Text.Trim());
Cheers!!