Linq to SQL use case and Lag equivalent - sql

Question

I have this SQL which I am trying to convert to LINQ , how can this be converted?
Is there an equivalent of Lag at all?
I see there is a case statement not sure how to use it
SELECT
ah.AuthHist_ID,
ah.F_ID,
CASE WHEN ah.AuthPIFlg = 1 OR ah.AuthPINVFlg = 1 THEN 'True' ELSE 'False' end AS chkReqPI,
lag(CASE WHEN ah.AuthPIFlg = 1 OR ah.AuthPINVFlg = 1 THEN 'True' ELSE 'False' end, 1, null) OVER (ORDER BY ah.f_id, ah.AuthHist_ID) AS prevChkReqPI,
ah.Cr8Dt,
lag(ah.Cr8Dt, 1, null) OVER (ORDER BY ah.f_id, ah.AuthHist_ID) AS prevCr8Dt,
cu.UserName AS Cr8UserName,
lag(cu.UserName, 1, null) OVER (ORDER BY ah.f_id, ah.AuthHist_ID) AS prevCr8UserName,
fh.UpdtDt,
FROM AuthHist Ah
LEFT JOIN User cu
ON Ah.Cr8User_ID = cu.User_ID
WHERE Ah.F_ID = #fid
return (from a in DbContext.AuthHist
join c DbContext.User on a.UpdtUserId equals c.UserId
where a.FId == fId
select new AuthHistEntity()
{
FId = a.FId,
checkReqPI = a.AuthPIflg = 1 || a.AuthPINVflg = 1 :
});

Answer 1

I have this SQL which I am trying to convert to LINQ , how can this be
converted?
You can use linq to do this, but it will be a bit more complicated.
For the case when in linq, you can directly use the ternary operator(?:) instead.
As for the Lag() function, we need to replace this function with another sql writing as follow：
(This is just to facilitate the understanding of the linq statement below, you do not need to change your sql statement)
select kh.AuthHist_ID,kh.F_ID,
CASE WHEN kh.AuthPIFlg = 1 OR kh.AuthPINVFlg = 1 THEN 'True' ELSE 'False' end AS chkReqPI,
CASE WHEN (ch.AuthPIFlg is null or ch.AuthPINVFlg is null) then Null when (ch.AuthPIFlg = 1 OR ch.AuthPINVFlg = 1) THEN 'True' ELSE 'False' end AS prevChkReqPI ,
kh.Cr8Dt,
ch.Cr8Dt AS prevCr8Dt ,
kh.UserName AS Cr8UserName,
cu.UserName AS prevCr8UserName
from
( select -1+row_number() over (order by ah.f_id,ah.AuthHist_ID) as row1, * from
(select * from AuthHist a left join User u on a.Cr8User_ID = u.User_ID) Ah) kh
left join (select * from AuthHist a left join User u on a.Cr8User_ID = u.User_ID) cu ON kh.row1 = cu.AuthHist_ID
left join AuthHist ch on kh.row1 = ch.AuthHist_ID where kh.F_ID =#fid
Because the types of some fields are uncertain, I have create the AuthHistEntity class as follow, you can modify some details according to your needs.
public class AuthHistEntity
{
public int AuthHist_ID { get; set; }
public int F_ID { get; set; }
public string chkReqPI { get; set; }
public string prevChkReqPI { get; set; }
public string Cr8Dt { get; set; }
public string prevCr8Dt { get; set; }
public string Cr8UserName { get; set; }
public string prevCr8UserName { get; set; }
}
Here is the linq writing, you can try it:
var newUser = (from a in DbContext.AuthHist
join u in DbContext.User on a.Cr8User_ID equals u.User_ID
select new { a, u }).ToList();
var newAuthList = (from t in newUser
select new
{
row_1 = t.a.AuthHist_ID - 1,
data = t
}).ToList();
int fid = 1;
var result = (from dt in
(from ch in newAuthList
join ah in DbContext.AuthHist on ch.row_1 equals ah.AuthHist_ID
into ot from otnew in ot.DefaultIfEmpty()
select new { T1 = ch, T2 = otnew == null ? new AuthHist() : otnew
}).ToList()
join nu in newUser on dt.T1.row_1 equals nu.a.AuthHist_ID
into yG from otnew in yG.DefaultIfEmpty()
where dt.T1.data.a.F_ID == fid
select new AuthHistEntity()
{
AuthHist_ID = dt.T1.data.a.AuthHist_ID,
F_ID = dt.T1.data.a.F_ID,
chkReqPI = (dt.T1.data.a.AuthPIFlg == 1 || dt.T1.data.a.AuthPINVFlg == 1) ? "True" : "False",
prevChkReqPI = (dt.T2.AuthPIFlg == null || dt.T2.AuthPINVFlg == null) ? null : ((dt.T2.AuthPIFlg == 1 || dt.T2.AuthPINVFlg == 1) ? "True" : "Flase"),
Cr8Dt = dt.T1.data.a.Cr8Dt,
prevCr8Dt = dt.T2.Cr8Dt == null ? null : dt.T2.Cr8Dt,
Cr8UserName = dt.T1.data.u.UserName,
prevCr8UserName = otnew == null ? null : otnew.u.UserName
}).ToList();