sql server左联接并且不存在的地方比较两个不同数据库中两个表之间的记录

Question

I have write a sql server script which will do the following

1. select records from one table1 in a database1 based on two fields like so

   select * from table1FromDB1 where ID = 5 and TYPE = "TeST"

2. I need to do another select on another different table like Table2 in a different database like DB2 perform the logic

If the records select from Table1 exist in Table2 take the one from Table2 , else if records in Table1 does not exist in table2 , use the one in table1

I am not good with left join or where not exist but I am told that is how to do it.

so here is my first attempt

select * from table1 t1
left join db2.table2 as t2 on t1.id = t2.id and t1.type = t2.type 
where not exists  (???) 
where ID = 5 and TYPE = "Test" 

Any help is appreaciated. Thanks.

Answer 1

With a LEFT JOIN, you will get all records from table 1 and the values from table 2 that are not matched through the ON conditions will return a NULL value. In your example, we want to use the value from table 1 when we see a NULL value from table 2. SQL Server has ISNULL() that can accomplish this. ISNULL() will check to see if the first value in the list is null, and if it is null, it will return the second value in the list. If the first value is not null, it will use the first value. For more than 2 values, consider COALESCE().

SELECT ISNULL(t2.yourvalue, t1.yourvalue)
FROM table1 t1
LEFT JOIN table2 t2
ON t1.id = t2.id 
AND t1.type = t2.type