Select Right('00000' + Cast(indent.indent_ID As VarChar(5)), 5) As 'Sr. No.',
  indent.financial_year,
  indent.[from],
  indent.[to],
  indent.indent_date,
  indent.menu,
  indent.required_date,
  indent_details.item_name,
  indent_details.item_UOM,
  indent_details.rate,
  CASE
	WHEN indent_details.qty_issued is NULL THEN (SELECT indent_details.qty_reqd  FROM indent_details)
	else  indent_details.qty_issued        
  end 'Quantity',
  CASE
	WHEN indent_details.qty_issued is NULL THEN (SELECT (indent_details.qty_reqd*indent_details.rate) FROM indent_details)
	else  (indent_details.qty_issued*indent_details.rate)          
  end 'Rate'
From indent Inner Join
  indent_details On indent.indent_ID = indent_details.IndentNumber

(1 row(s) affected)

Server: Msg 512, Level 16, State 1, Line 1
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Select Right('00000' + Cast(indent.indent_ID As VarChar(5)), 5) As 'Sr. No.',
  indent.financial_year,
  indent.[from],
  indent.[to],
  indent.indent_date,
  indent.menu,
  indent.required_date,
  indent_details.item_name,
  indent_details.item_UOM,
  indent_details.rate,
  CASE
	WHEN indent_details.qty_issued is NULL THEN indent_details.qty_reqd
	else  indent_details.qty_issued        
  end 'Quantity',
  CASE
	WHEN indent_details.qty_issued is NULL THEN (indent_details.qty_reqd*indent_details.rate)
	else  (indent_details.qty_issued*indent_details.rate)          
  end 'Rate'
From indent Inner Join
  indent_details On indent.indent_ID = indent_details.IndentNumber