select results from two tables but choose one field over another - sql

Question

I have two tables.
tblEmployee tblExtraOrMissingInfo
id nvarchar(10) id nvarchar(10)
Name nvarchar(50) Name nvarchar(50)
PreferredName nvarchar(50)
UsePreferredName bit
The data (brief example)
tblEmployee tblExtraOrMissingInfo
id Name id Name PreferredName UsePreferredName
AB12 John PN01 Peter Tom 1
LM22 Lisa YH76 Andrew Andy 0
PN01 Peter LM22 Lisa Liz 0
LK655 Sarah
I want a query to produce the following result
id Name
AB12 John
LM22 Lisa
PN01 Tom
YH76 Andrew
LK655 Sarah
So what I want is all the records from tblEmployee returned and any records in tblExtraOrMissingInfo that are not already in tblEmployee.
If there is a record in both tables with the same id I would like is if the UsePreferredName field in tblExtraOrMissingInfo is 1 for the PreferredName to be used rather than the Name field in the tblEmployee, please see the record PN01 in the example above.

Answer 1

It is slightly faster to use a left join and coalesce than to use the case statement (most servers are optimized for coalesce).
Like this:
SELECT E.ID, COALESCE(P.PreferredName,E.Name,'Unknown') as Name
FROM tblemployee E
LEFT JOIN tblExtraOrMissingInfo P ON E.ID = P.ID AND P.UsePreferredName = 1
The ,'Unknown' is not needed to answer your question, but I added
here to show that you can enhance this query to handle cases where the
name is not available in both tables and you don't want nulls in your result

Answer 2

left join on the employee table and use a case expression for name.
select e.id
,case when i.UsePreferredName = 1 then i.PreferredName else e.name end as name
from tblemployee e
left join tblExtraOrMissingInfo i on i.id=e.id

Answer 3

You can LEFT OUTER JOIN the tables together, then use a CASE statement and COALESCE() formula to get this:
SELECT
tblEmployee.id,
CASE WHEN UsePreferredName = 1 THEN COALESCE(PreferredName, tblEmployee.Name) ELSE tblEmployee.name
FROM
tblEmployee
LEFT OUTER JOIN tblExtraOrMissingInfo
ON tblEmployee.id = tblExtraOrMissingInfo.id
CASE checks that usePreferredName value, and the COALESCE() then grabs PreferredName unless it's NULL. Then it grabs the employees name from tblEmployee.

Answer 4

select id ,name
from
(
select id ,name from tblEmployee
union all
select id ,name from tblExtraOrMissingInfo
)group by id,name

Answer 5

This is an other way you can achieve the results. You can also use CTE to get the same result.
CREATE TABLE #tblEmployee
(
id NVARCHAR(10)
, Name NVARCHAR(50)
);
CREATE TABLE #tblExtraOrMissingInfo
(
id NVARCHAR(10)
, Name NVARCHAR(50)
, PreferredName NVARCHAR(50)
, UsePreferredName BIT
);
INSERT INTO #tblEmployee
( id, Name )
VALUES ( N'AB12' -- id - nvarchar(10)
, N'John' -- Name - nvarchar(50)
),
( N'LM22' -- id - nvarchar(10)
, N'Lisa' -- Name - nvarchar(50)
),
( N'PN01' -- id - nvarchar(10)
, N'Peter' -- Name - nvarchar(50)
),
( N'LK655' -- id - nvarchar(10)
, N'Sarah' -- Name - nvarchar(50)
);
INSERT INTO #tblExtraOrMissingInfo
( id, Name, PreferredName, UsePreferredName )
VALUES ( N'PN01' -- id - nvarchar(10)
, N'Peter' -- Name - nvarchar(50)
, N'Tom' -- PreferredName - nvarchar(50)
, 1 -- UsePreferredName - bit
),
( N'YH76' -- id - nvarchar(10)
, N'Andrew' -- Name - nvarchar(50)
, N'Andy' -- PreferredName - nvarchar(50)
, 0 -- UsePreferredName - bit
),
( N'LM22' -- id - nvarchar(10)
, N'Lisa' -- Name - nvarchar(50)
, N'Liz' -- PreferredName - nvarchar(50)
, 0 -- UsePreferredName - bit
);
SELECT R.id
, CASE WHEN tbl.UsePreferredName = 1 THEN tbl.PreferredName
ELSE R.Name
END AS NAME
FROM #tblExtraOrMissingInfo tbl
RIGHT JOIN ( SELECT id
, Name
FROM #tblEmployee
UNION
SELECT id
, Name
FROM #tblExtraOrMissingInfo
) AS R
ON R.id = tbl.id
AND R.Name = tbl.Name;