how to avoid duplicate data in sql command - mysql

Question

I try to use DISTINCT to avoid duplication of the data, but no work.
How to avoid the duplicate data?
Table 1
Employee code Deduction Voucher no Dec_Amount
001 999 50
001 888 20
002 777 100
Table 2
Employee code Payslip Voucher No Pay_Amount
001 111 100
002 222 200
The output should be:
Employee code Deduction Voucher no Dec_Amount Payslip Voucher No Pay_Amount
001 999 50 111 100
001 888 20
002 777 100 222 200
But i got the table like this.
Employee code Deduction Voucher no Dec_Amount Payslip Voucher No Pay_Amount
001 999 50 111 100
001 888 20 111 100
002 777 100 222 200

Answer 1

You cannot get those results with just a SQL query. It seems to me you need it in this format for display in a table/excel spreadsheet. If this is the case you would have to handle the "Hiding" of the particular entries with some other code. The reason is because the entries you want to hide are correctly associated with the 001 employee.

Answer 2

While I do agree this probably makes a lot more sense to do in your front end code, it is possible to do in SQL. Using a variable you get a function similar to SQL Server's ROW_NUMBER function and then only join on the first row per employee code.
See the sqlfiddle - http://sqlfiddle.com/#!2/47302/11
SELECT t1.`Employee code`,`Deduction Voucher no`,`Dec_Amount`,
COALESCE(`Payslip Voucher No`,'') as `Payslip Voucher No`,
COALESCE(CAST(`Pay_Amount` as char(10)),'') as `Pay_Amount`
FROM Table2 t2
RIGHT JOIN
(
SELECT #row_num := IF(#prev_value=`Employee code`,#row_num+1,1) AS RowNumber
,`Employee code`,`Deduction Voucher no`,`Dec_Amount`
,#prev_value := `Employee code`
FROM Table1,
(SELECT #row_num := 1) x,
(SELECT #prev_value := '') y
ORDER BY `Employee code`
) t1
ON t1.`Employee code`=t2.`Employee code` AND t1.RowNumber=1

Answer 3

To expand on #d.lanza38's answer, there is no way given for the DB to tell which row in table1 should get the data from table2. Remember that there is no order to the data in the database, so there is no inherent concept of "the first row with employee code 001".
A standard (inner) join will put them as you have shown - on both. Which is actually correct - your table structures say that for every payslip in table2, there can be many deductions. So if you want the data from both tables, the deductions have to have the matching payslip data attached.
You can't use DISTINCT to magically fix your data - you need to understand the data structures, and relate them correctly.
To get what is in your example (which may be wrong) try this SQL:
select
a.Employee_code,
Deduction_Voucher_no,
Dec_Amount,
Payslip_Voucher_No,
Pay_Amount
from
table1 as a
inner join table2 as b on a.employee_code = b.employee_code
where Deduction_Voucher_no = (
select max(Deduction_Voucher_no)
from table1 as c
where a.Employee_code = c.Employee_code)
UNION
select
a2.Employee_code,
Deduction_Voucher_no,
Dec_Amount,
null as Payslip_Voucher_No,
null as Pay_Amount
from
table1 as a2
inner join table2 as b2 on a2.employee_code = b2.employee_code
where Deduction_Voucher_no <> (
select max(Deduction_Voucher_no)
from table1 as c2
where a2.Employee_code = c2.Employee_code)
order by 1,2 desc
Note: untested, because I don't have your database, and don't even know which database engine you are using. If it complains about selecting nulls, replace with 0 or '' depending upon the data type.
UPDATE improved SQL and provided a fiddle: http://sqlfiddle.com/#!2/e7fc2/2