Find The First Row

,

Find the First Row

Recently I was asked an interesting programming question about how to return certain

information. At first glance, it appeared to be simple to me, but a few people had

the same problem. After a little testing, I found it was interesting and decided to make

some notes:

The Problem

A colleague had a table that contained information about employees. Some department number, a name,

and a code. I built a short test table using the following code:

create table MyTest
( id int,
  nm char( 10),
  code int
)

The challenge in this instance was to return the first employee from each department. Suppose

the table contained the following information:

ID	Nm		Code
---	--------	------
1	Joe       	550	
1	Jim       	600	
2	Mary      	300	
3	Mike      	350	
4	Pam       	50	
4	Jeff      	200	
4	Judge     	100	

The idea was to return the follwing results:

ID	Nm		Code
---	--------	------
1	Joe       	550	
2	Mary      	300	
3	Mike      	350	
4	Pam       	50	

The Solution

There are actually a few solutions, as there are for most problems in SQL. To return the

desired results, we want to return the first instance of each id value. The easiest

way to do this, IMHO, is to use a correlated subquery.

In constructing this query, we first build our column list as a simple select:

select id
	,nm
	,code
 from MyTest a

Now we can build a select that finds the first name. We cannot use the MIN()

function for the following reason. Suppose we run:

select id, min( nm) 
 from MyTest b
 group by id

we get

1	Jim       
2	Mary      
3	Mike      	
4	Jeff      	

The problem is with department 4. The first instance is Pam, not Jeff. Using an ordering

clause also causes a similar problem. What we want is the first item in Natural order, or

the order in which the items are stored.

This is a perfect place for the TOP operator. We can use this operator to limit our

result set, and without an ORDER BY clause or other operator to affect the result set, the

natural order takes over. The following code:

select top 1 nm
 from MyTest b
 where id = 4

we get


which is what we want.

NOTE: The example is returning the natural order of the data, which is what was requested.

The presence of an index can change the ordering of the data. An ORDER by clause can be

used to force the first row based on a column of data, but there must be such a column

available for ordering.

We can now link these two queries together, making the second a subquery of the first as follows:

select id
	,nm
	,code
 from MyTest a
 where nm = ( select top 1 nm 
					from MyTest b
					where b.id = a.id
				)

Notice that the outer query (the first query) has a reference embedded in the subquery (the

second query). This reference makes the correlation between the subquery and the outer

query. This query gives us the results we desire.

ID	Nm		Code
---	--------	------
1	Joe       	550	
2	Mary      	300	
3	Mike      	350	
4	Pam       	50	

Code used to create this article is here: findfirst.sql

Conclusions

I didn't think this was a difficult query, but it is a little tricky. A couple

years ago, however, it may have stumped be for awhile. Having a broad knowledge of the

different functions in T-SQL can come in extremely handy in devising solutions to a query problem.

Hopefully this has helped you and as always, I welcome feedback on this topic.

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