SQL/mysql - Select distinct/UNIQUE but return all columns?

Question

SELECT DISTINCT field1, field2, field3, ......   FROM table

I am trying to accomplish the following sql statement but I want it to return all columns is this possible? Something like:

SELECT DISTINCT field1, * from table

Solution 1

You're looking for a group by:

select *
from table
group by field1

Which can occasionally be written with a distinct on statement:

select distinct on field1 *
from table

On most platforms, however, neither of the above will work because the behavior on the other columns is unspecified. (The first works in MySQL, if that's what you're using.)

You could fetch the distinct fields and stick to picking a single arbitrary row each time.

On some platforms (e.g. PostgreSQL, Oracle, T-SQL) this can be done directly using window functions:

select *
from (
   select *,
          row_number() over (partition by field1 order by field2) as row_number
   from table
   ) as rows
where row_number = 1

On others (MySQL, SQLite), you'll need to write subqueries that will make you join the entire table with itself (example), so not recommended.

Solution 2

From the phrasing of your question, I understand that you want to select the distinct values for a given field and for each such value to have all the other column values in the same row listed. Most DBMSs will not allow this with neither DISTINCT nor GROUP BY, because the result is not determined.

Think of it like this: if your field1 occurs more than once, what value of field2 will be listed (given that you have the same value for field1 in two rows but two distinct values of field2 in those two rows).

You can however use aggregate functions (explicitely for every field that you want to be shown) and using a GROUP BY instead of DISTINCT:

SELECT field1, MAX(field2), COUNT(field3), SUM(field4), ....
FROM table GROUP BY field1

Solution 3

If I understood your problem correctly, it's similar to one I just had. You want to be able limit the usability of DISTINCT to a specified field, rather than applying it to all the data.

If you use GROUP BY without an aggregate function, which ever field you GROUP BY will be your DISTINCT filed.

If you make your query:

SELECT * from table GROUP BY field1;

It will show all your results based on a single instance of field1.

For example, if you have a table with name, address and city. A single person has multiple addresses recorded, but you just want a single address for the person, you can query as follows:

SELECT * FROM persons GROUP BY name;

The result will be that only one instance of that name will appear with its address, and the other one will be omitted from the resulting table. Caution: if your fileds have atomic values such as firstName, lastName you want to group by both.

SELECT * FROM persons GROUP BY lastName, firstName;

because if two people have the same last name and you only group by lastName, one of those persons will be omitted from the results. You need to keep those things into consideration. Hope this helps.

Solution 4

That's a really good question. I have read some useful answers here already, but probably I can add a more precise explanation.

Reducing the number of query results with a GROUP BY statement is easy as long as you don't query additional information. Let's assume you got the following table 'locations'.

--country-- --city--
 France      Lyon
 Poland      Krakow
 France      Paris
 France      Marseille
 Italy       Milano

Now the query

SELECT country FROM locations
GROUP BY country

will result in:

--country--
 France
 Poland
 Italy

However, the following query

SELECT country, city FROM locations
GROUP BY country

...throws an error in MS SQL, because how could your computer know which of the three French cities "Lyon", "Paris" or "Marseille" you want to read in the field to the right of "France"?

In order to correct the second query, you must add this information. One way to do this is to use the functions MAX() or MIN(), selecting the biggest or smallest value among all candidates. MAX() and MIN() are not only applicable to numeric values, but also compare the alphabetical order of string values.

SELECT country, MAX(city) FROM locations
GROUP BY country

will result in:

--country-- --city--
 France      Paris
 Poland      Krakow
 Italy       Milano

or:

SELECT country, MIN(city) FROM locations
GROUP BY country

will result in:

--country-- --city--
 France      Lyon
 Poland      Krakow
 Italy       Milano

These functions are a good solution as long as you are fine with selecting your value from the either ends of the alphabetical (or numeric) order. But what if this is not the case? Let us assume that you need a value with a certain characteristic, e.g. starting with the letter 'M'. Now things get complicated.

The only solution I could find so far is to put your whole query into a subquery, and to construct the additional column outside of it by hands:

SELECT
     countrylist.*,
     (SELECT TOP 1 city
     FROM locations
     WHERE
          country = countrylist.country
          AND city like 'M%'
     )
FROM
(SELECT country FROM locations
GROUP BY country) countrylist

will result in:

--country-- --city--
 France      Marseille
 Poland      NULL
 Italy       Milano

Solution 5

SELECT  c2.field1 ,
        field2
FROM    (SELECT DISTINCT
                field1
         FROM   dbo.TABLE AS C
        ) AS c1
        JOIN dbo.TABLE AS c2 ON c1.field1 = c2.field1

View more solutions

Comments

[Conrad Frix]

I think you forgot an alias row_number() over (partition by field1) row_number

[Ankur-m]

That won't do the job. You have selected the distinct column in the subquery but the where clause gets all those columns with that value. So the query is as good as writing 'select * from table' unless 'field' column is a unique column in which case the distinct on that column isn't required at all.

[Ankur-m]

The query won't parse for me and gives an error: The ranking function "row_number" must have an ORDER BY clause. We need to add order by clause after partition by field1. So the correct query will be select * from ( select *, row_number() over (partition by field1 order by orderbyFieldName) as row_number from table ) as rows where row_number = 1

[Robbert]

To make this a good answer, you should include a little more detail about what you mean.

[stalk]

+1 for this solution. So we can do SELECT field1, MIN(field2), MIN(field3), MIN(field4), .... FROM table GROUP BY field1, and field2, 3, 4,,, are not required to be integers (or other digits), they can be char fields as well

[Joaquin Iurchuk]

Thanks! I was in the same problem and the solution was the GROUP BY

[Viuu -a]

GROUP BY clause must match selected fields. else it will throw error like filed2 must appear in the GROUP BY clause or be used in an aggregate function

[signonsridhar]

Was working nicely until I got stuck at a boolean column. MIN(Dynamic) column values get modified to false even if it was true.. Any other aggregate function available to address boolean – signonsridhar 6 mins ago. Sum(dynamic) changed false to 1

[Garrett Simpson]

This does not answer the question, the OP was trying to get all the data of the table but remove rows containing duplicates of a single field

[Garrett Simpson]

This does not answer the question, the OP was trying to get all the data of the table but remove rows containing duplicates of a single field

[Garrett Simpson]

As mentioned in the accepted answer, would work for most incarnations of SQL -- only for MYSQL

[Garrett Simpson]

too complicated and specific to one implementation of SQL

[Garrett Simpson]

As mentioned in the accepted answer, would work for most incarnations of SQL -- only for MYSQL

[Garrett Simpson]

Great suggestion, led me to my solution which I think is more universal -- take a look!

[meta4]

Also in Oracle (Oracle SQL Developer) you can not specify select *, row_number() over (partition by field1 order by field2) as row_number from table. You have to explicitly use table name/alias in select query select **table**.*, row_number() over (partition by field1 order by field2) as row_number from table

[Talha]

Why there is C alias when it can work without it? in line FROM dbo.TABLE AS C

[jarlh]

"Answers to questions tagged with SQL should use ISO/IEC standard SQL."

[Denis de Bernardy]

@jarlh: Might be ... today. As you may notice, this answer is almost 7 years old, a point in time where that wasn't the case insofar as I can recollect from back when I was active. You're welcome to retag and/or edit the answer if you feel it's necessary.

[Stormy]

I believe this is due to my use of RedGate SQLPrompt. The way I have it configured, it always adds aliases - even if unnecessary. It's there "just in case"

[Drew]

@signonsridhar cast your boolean to an int and use sum; e.g. sum(cast(COL as int)) > 0

[Dr. House]

select distinct on field1 * from table; This doesn't seem like correct syntax for mysql - at least no anymore.

[Shin Kim]

It is not different with SELECT * FROM table;. Even more It is slow.

[Sherif]

Please, try your answer first.

[Chilianu Bogdan]

select distinct on (field1) * from table; works also in PostgreSQL

[Brandon Printiss]

This worked for me!! It's worth noting tho, if you are using fetch_array() then you will need to call each row via an index label rather than implicitly calling the row name. There aren't enough characters in this for me to write out the example I have :X sorry!!

[Michael Fever]

This looked promising for me but it still brought back all the rows, not the distinct field1. :(

[Michael Fever]

Works for MSSQL

[NehaK]

Cannot group on fields selected with '*'.

[coderboi]

i think this only works in mysql, not postgresql

[coderboi]

@ChilianuBogdan thank you, you are a life saver! This should be the answer right here. Very compact.

[ddruganov]

doesnt work in mysql5.5, gives: Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column