How to select static values on mysql select query? - mysql

Question

I am new to mysql, here i am trying to get data from database table.
select id,txnid,amount,status from txn_details;
With above query Getting data successfully but status column getting 0 or 1 or 2, but i want 0 as failed, 1 as success and 2 as not processed.
How to change my query?

Answer 1

You can use a case
select id, txnid, amount,
case when status = 0 then 'failed'
when status = 1 then 'success'
else 'not processed'
end as status
from txn_details;

Answer 2

We can use an expression in the SELECT list. It could be a searched CASE expression e.g.
SELECT CASE t.status
WHEN 0 THEN 'failed'
WHEN 1 THEN 'success'
WHEN 2 THEN 'not processed'
ELSE 'unknown'
END AS status_name
, t.status
, t.amount
, t.txnid
FROM txn_details t
This approach is ANSI-92 standards compliant, and will work in most relational databases.
There are some other MySQL specific alternatives, such as the ELT function ...
SELECT ELT(t.status+1,'failed','success','not processed') AS status_name
, t.status
, t.amount
, t.txnid
FROM txn_details t
https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_elt

Answer 3

If you prefer a central point of maintenance (ie you prefer not to recode all your queries when a new status comes along) you could create a status table and either use a join or sub query to get the values, alternatively you could create a function, for example
drop table if exists txn_details,txn_status;
create table txn_details(id int, txnid int, amount int , status int);
insert into txn_details values
(1,1,10,1),(2,1,10,2),(3,1,10,4);
create table txn_status (id int, statusval varchar(20));
insert into txn_status values
(1,'success'),(2,'not processed'), (3,'failed');
drop function if exists f;
delimiter $$
create function f(instatus int)
returns varchar(20)
begin
declare rval varchar(20);
return (select
case when instatus = 1 then 'success'
when instatus = 2 then 'not processed'
when instatus = 3 then 'failed'
else 'Unknown'
end
);
select t.*,coalesce(ts.statusval,'Unknown') status
from txn_details t
left join txn_status ts on ts.id = t.status;
select t.*,coalesce((select statusval from txn_status ts where ts.id = t.status),'Unknown') status
from txn_details t;
Note the use of coalesce in case a status is not found.
Both produce this result
+------+-------+--------+--------+---------------+
| id | txnid | amount | status | status |
+------+-------+--------+--------+---------------+
| 1 | 1 | 10 | 1 | success |
| 2 | 1 | 10 | 2 | not processed |
| 3 | 1 | 10 | 4 | Unknown |
+------+-------+--------+--------+---------------+
3 rows in set (0.00 sec)
Using the function like this
select t.*, f(status) as status
from txn_details t;
also produces the same result.
Of course using a status table or a function means you have to communicate their availability and enforce their use.
I would also consider the using a foreign key constraint in txn_details to cut down on the number of unknown values and put procedures in place to stop people adding new status codes at will without going through change control

Answer 4

The following query would work. It uses CASE ... END to determine and return values for the virtual column status.
SELECT id,txnid,amount,
CASE
WHEN status = 0 THEN 'failed'
WHEN status = 1 THEN 'success'
WHEN status= 2 THEN 'not processed'
END AS status
FROM txn_details;