Question

I’m writing an sql statement (see below) which compares 2 serial numbers on 2 different tables, table1 and table2. If the serial number exists then i want another column on table1 to be updated with a ‘yes’ and if the serial number in table1 does not exist in table2 I want to update with a ‘no’

My current sql statement works perfectly and fills the column with ‘yes’ when necessary. My problem is that instead of placing a ‘no’ when the serial number doesnt exist, instead it updates to NULL. My where statement is important as I only want to compare serial numbers on both tables where the date and install match up in both tables. I do not want to compare every row. Thanks

UPDATE dbo.table1
    SET [Match] = CASE WHEN dbo.table2.[Serial Number] IS NOT NULL
    THEN 'yes' ELSE 'no' END
        FROM dbo.table1 LEFT OUTER JOIN dbo.table2
            ON dbo.table2.[Serial Number] = dbo.table1.[Serial Number]
        Where dbo.table1.[Date] = 'JAN11' AND 
              dbo.table1.[Install] = 'new' AND
              dbo.table2.[Date] = 'JAN11' AND 
              dbo.table2.[Install] = 'new'

Answer 1

Put the WHERE conditions to JOIN’s condition:

UPDATE dbo.table1
SET [Match] = CASE WHEN dbo.table2.[Serial Number] IS NOT NULL
THEN 'yes' ELSE 'no' END
    FROM dbo.table1 LEFT OUTER JOIN dbo.table2
        ON dbo.table2.[Serial Number] = dbo.table1.[Serial Number]
    AND dbo.table1.[Date] = 'JAN11' AND 
          dbo.table1.[Install] = 'new' AND
          dbo.table2.[Date] = 'JAN11' AND 
          dbo.table2.[Install] = 'new'

---

@Baz1nga:

The WHERE causes the rows to be filtered out, hence the setting of value(s) are not being assigned back to table1.

Try this:

create table tblA
(
x int, y varchar(10)
);


create table tblB
(
x int, some_date varchar(10), serial_number int
);


insert into tblA(x,y) values(1,'www');
insert into tblB(x,some_date,serial_number) values(1,'yyy',76);

print 'First update';
update tblA
set y = case when tblB.serial_number is not null then 'Yes' else 'No' end
from tblA 
left join tblB on tblB.x = tblA.x 
where tblB.some_date = 'JAN11';

select * from tblA;

print 'Second update';
update tblA
set y = case when tblB.serial_number is not null then 'Yes' else 'No' end
from tblA 
left join tblB on tblB.x = tblA.x 
and tblB.some_date = 'JAN11';

select * from tblA;

Output:

(1 row(s) affected)

(1 row(s) affected)
First update

(0 row(s) affected)
x           y
----------- ----------
1           www

(1 row(s) affected)

Second update

(1 row(s) affected)
x           y
----------- ----------
1           No

(1 row(s) affected)