How to select the top n from a union of two queries where the resulting order needs to be ranked by individual query? - sql

Question

Let's say I have a table with usernames:
Id | Name
-----------
1 | Bobby
20 | Bob
90 | Bob
100 | Joe-Bob
630 | Bobberino
820 | Bob Junior
I want to return a list of n matches on name for 'Bob' where the resulting set first contains exact matches followed by similar matches.
I thought something like this might work
SELECT TOP 4 a.* FROM
(
SELECT * from Usernames WHERE Name = 'Bob'
UNION
SELECT * from Usernames WHERE Name LIKE '%Bob%'
) AS a
but there are two problems:
It's an inefficient query since the sub-select could return many rows (looking at the execution plan shows a join happening before top)
(Almost) more importantly, the exact match(es) will not appear first in the results since the resulting set appears to be ordered by primary key.
I am looking for a query that will return (for TOP 4)
Id | Name
---------
20 | Bob
90 | Bob
(and then 2 results from the LIKE query, e.g. 1 Bobby and 100 Joe-Bob)
Is this possible in a single query?

Answer 1

You could use a case to place the exact matches on top:
select top 4 *
from Usernames
where Name like '%Bob%'
order by
case when Name = 'Bob' then 1 else 2 end
Or, if you're worried about performance and have an index on (Name):
select top 4 *
from (
select 1 as SortOrder
, *
from Usernames
where Name = 'Bob'
union all
select 2
, *
from Usernames
where Name like '%Bob%'
and Name <> 'Bob'
and 4 >
(
select count(*)
from Usernames
where Name = 'Bob'
)
) as SubqueryAlias
order by
SortOrder

Answer 2

A slight modification to your original query should solve this. You could add in an additional UNION that matches WHERE Name LIKE 'Bob%' and give this priority 2, changing the '%Bob' priority to 3 and you'd get an even better search IMHO.
SELECT TOP 4 a.* FROM
(
SELECT *, 1 AS Priority from Usernames WHERE Name = 'Bob'
UNION
SELECT *, 2 from Usernames WHERE Name LIKE '%Bob%'
) AS a
ORDER BY Priority ASC

Answer 3

This might do what you want with better performance.
SELECT TOP 4 a.* FROM
(
SELECT TOP 4 *, 1 AS Sort from Usernames WHERE Name = 'Bob'
UNION ALL
SELECT TOP 4 *, 2 AS Sort from Usernames WHERE Name LIKE '%Bob%' and Name <> 'Bob'
) AS a
ORDER BY Sort

Answer 4

This works for me:
SELECT TOP 4 * FROM (
SELECT 1 as Rank , I, name FROM Foo WHERE Name = 'Bob'
UNION ALL
SELECT 2 as Rank,i,name FROM Foo WHERE Name LIKE '%Bob%'
) as Q1
ORDER BY Q1.Rank, Q1.I

Answer 5

SET ROWCOUNT 4
SELECT * from Usernames WHERE Name = 'Bob'
UNION
SELECT * from Usernames WHERE Name LIKE '%Bob%'
SET ROWCOUNt 0

Answer 6

The answer from Will A got me over the line, but I'd like to add a quick note, that if you're trying to do the same thing and incorporate "FOR XML PATH", you need to write it slightly differently.
I was specifying XML attributes and so had things like :
SELECT Field_1 as [#attr_1]
What you have to do is remove the "#" symbol in the sub queries and then add them back in with the outer query. Like this:
SELECT top 1 a.SupervisorName as [#SupervisorName]
FROM
(
SELECT (FirstNames + ' ' + LastName) AS [SupervisorName],1 as OrderingVal
FROM ExamSupervisor SupervisorTable1
UNION ALL
SELECT (FirstNames + ' ' + LastName) AS [SupervisorName],2 as OrderingVal
FROM ExamSupervisor SupervisorTable2
) as a
ORDER BY a.OrderingVal ASC
FOR XML PATH('Supervisor')
This is a cut-down version of my final query, so it doesn't really make sense, but you should get the idea.