How can I select only the first distinct match from a field in MySQL?

Question

How can I only return the first distinct match of a field in MySQL?

My Table:

name     hash
----------------
Anna     ABC
Barb     DEF
Charlie  GHI
A         


        

Answer 1

DISTINCT does not work that way, the values must be distinct across all columns being returned.

You can always use an aggregate function on the hash function and GROUP BY name which will return one hash value for each name:

SELECT name, min(hash) hash
FROM my_table 
WHERE name LIKE '%An%' 
GROUP BY name;

See SQL Fiddle with Demo.

Note: using the aggregate function with the GROUP BY will make sure that you will always return the expected value for the hash column. When you do not GROUP BY or aggregate the items in the SELECT list, you might return unexpected results. (see MySQL Extensions to GROUP BY)

From the MySQL Docs:

MySQL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. ... You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However, this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group, so unless they are the same, the values chosen are indeterminate. Furthermore, the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen, and ORDER BY does not affect which values the server chooses.

Answer 2

try this

SELECT name , hash FROM my_table WHERE name LIKE '%An%'
GROUP BY name;

DEMO SQLFIDDLE HERE

Answer 3

When using GROUP BY, MySQL destroy the desc order on the same query level.

Instead of:

SELECT name, hash 
FROM my_table
GROUP BY name
ORDER BY name ASC, hash DESC

Use sub query on descending order:

SELECT * FROM(
  SELECT name, hash
  FROM my_table
  ORDER BY name ASC, hash DESC
)Q 
GROUP BY name

Answer 4

DISTINCT provides unique rows of data. In your example hash is different, hence why you are not getting the results you want.

Question: What is hash used for? Do you need it to unique, or is it not needed?

If you do not need it, remove it from the SELECT clause and you will have unique names.

If you need it, but it does not need to be unique, you can add a GROUP BY clause, i.e. GROUP BY name which will group your results by name (giving you only unique names). Note, using GROUP BY means that the value of hash could be either 'ABC' or 'JKL' when name is 'Anna'.

Answer 5

SELECT 
  name, hash 
FROM 
  my_table 
WHERE 
  name LIKE '%An%' 
GROUP BY 
  name;