How to insert using different table based on condition in same query - sql

Question

I am merging data in one table from tables of 2 database. Structure is as per below:
Table in new Database :
User Table : {UserName,Email}
Table in Database1 :
User Table : {UserName,Email,LastLogin}
Table in Database2 :
User Table : {UserName,Email,LastLogin}
Now i need to write query that if Email address are same in 2 tables from database 1 and database2 then we need to insert record where LastLogin is latest.
Can someone suggest over this.

Answer 1

I think you are in need of this.. :)
Try modifying it accordingly..
declare #Email_1 nvarchar(100),#Email_2 nvarchar(100),#UserName nvarchar(100),#Lastlogin_1 datetime,#Lastlogin_2 datetime,#loop int=0
use [Database1]
while #loop != (select count(Distinct Email) from [User Table])
BEGIN
use [Database1]
set #Email_1 = (select Distinct Email from [User Table] order by email asc offset #loop rows fetch next 1 rows only)
set #LastLogin_1 = (select top 1 max(LastLogin) from [User Table] where email=#Email_1)
use [Database2]
set #Email_2 = (select top 1 Email from [User Table] where Email like '%#Email_1%')
set #LastLogin_2 = (select top 1 max(LastLogin) from [User Table] where email=#Email_2)
if #email_1=#email_2
BEGIN
if #LastLogin_1>#LastLogin_2
BEGIN
use [Database_1]
set #username = (select top 1 Username from [user table] where email=#email_1 and lastlogin=#Lastlogin_1)
use [New Database]
insert into [User Table]
select #username,#email_1
END
else if #LastLogin_1<#LastLogin_2
BEGIN
use [Database_2]
set #username = (select top 1 Username from [user table] where email=#email_2 and lastlogin=#Lastlogin_2)
use [New Database]
insert into [User Table]
select #username,#email_1
use [Database1]
END
END
set #loop=#loop+1
END

Answer 2

My following code are assuming all tables are in one database, this is more for demo convenience purpose. In real world, when you have tables in different databases, then you need to use 3-part naming convention, i.e.
[DB].[Schema].[Table]
Also I am testing in sql server environment.
use tempdb
drop table dbo.merge_tbl, dbo.t1, dbo.t2;
create table dbo.merge_tbl (username varchar(30), email varchar(30));
create table dbo.t1 (username varchar(30), email varchar(30), lastlogin datetime)
create table dbo.t2 (username varchar(30), email varchar(30), lastlogin datetime)
go
-- insert a few records to tables
insert into dbo.t1 (username, email, lastlogin)
values ('james1', 'j#a.com', '20161001'), ('jenny1', 'j2#b.com', '20161002'), ('jeffrey1', 'j3#c.com', '20150101')
insert into dbo.t2(username, email, lastlogin)
values ('james2', 'j#a.com', '20161006'), ('jenny2', 'j2#b.com', '20151002'), ('jeffrey2', 'j4#c.com', '20170101')
go
-- this is the insert statement
insert into dbo.merge_tbl (username, email)
select case when t1.lastlogin >= t2.lastlogin then t1.username else t2.username end
, case when t1.lastlogin >= t2.lastlogin then t1.email else t2.email end
from dbo.t1
inner join dbo.t2
on t1.email = t2.email;
go
-- check result
select * from dbo.merge_tbl
Here is the result

Answer 3

follow 3 part naming convention([Database Name].[Schema].[Table name]) while accessing your table in different database. Also there may be multiple log in entries for a single user in the table [User Table] ,so use grouping function in such scenario.
If you are using SQL Server,use the below script for achieving the result.
INSERT INTO [New Database].dbo.[User Table] (UserName,Email)
SELECT CASE WHEN MAX(a.LastLogin)>=MAX(b.LastLogin) THEN a.[UserName] ELSE b.[UserName] END [UserName],a.Email
FROM Database1.dbo.[User Table] a
JOIN Database2.dbo.[User Table] b
ON a.Email=b.Email
GROUP BY a.[UserName],b.[UserName],a.Email

Answer 4

Assuming the three database db0, db1 and db2. For merging data in database db0 run the following query:
insert into db0.user('name','email')
select name, email from (
select db1.name,db1.email,
case
when db1.user.lastlogin >= db2.user.lastlogin
then db1.user.lastlogin
else db2.user.lastlogin
end as lastlogin
from db1.user,db2.user
where db1.user.email = db2.user.email ) as a
UPDATE
insert into db0.user(all columns)
select * from (
select "apply case for some columns" from db1.user,db2.user
where "apply your condition" ) as a
consider the following facts:
1. In this above query we are doing cross multiplication between db1.user and db2.user by which we get all possible values from these 2 tables
If you want to apply condition in "where" clause for email(for example db1.user.email = db2.user.email) then you can write db1.user.email or db2.user.email after "select", because both values are same.
If you want to apply >= or <= condition you have to apply "case" after "select".Because you have to fetch any one value from 2 tables
For example you want to fetch lastlogin data, for this condition I have applied case in lastlogin column. By this condition you will get column value from either db1.user or db2.user.
conditions in where clause can be written in case form after "select" also.
Please have a look on the UPDATE portion.
And I went for prayer, that is why this late reply.
If it helps mark as right answer.
Thank you