SQL/mysql - Select distinct/UNIQUE but return all

问题:

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

回答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.

回答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

回答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.

回答4:

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

回答5:

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

回答6:

Great question @aryaxt -- you can tell it was a great question because you asked it 5 years ago and I stumbled upon it today trying to find the answer!

I just tried to edit the accepted answer to include this, but in case my edit does not make it in:

If your table was not that large, and assuming your primary key was an auto-incrementing integer you could do something like this:

SELECT 
  table.*
FROM table
--be able to take out dupes later
LEFT JOIN (
  SELECT field, MAX(id) as id
  FROM table
  GROUP BY field
) as noDupes on noDupes.id = table.id
WHERE
  //this will result in only the last instance being seen
  noDupes.id is not NULL

回答7:

You can do it with a WITH clause.

For example:

WITH c AS (SELECT DISTINCT a, b, c FROM tableName)
SELECT * FROM tableName r, c WHERE c.rowid=r.rowid AND c.a=r.a AND c.b=r.b AND c.c=r.c

This also allows you to select only the rows selected in the WITH clauses query.

回答8:

For SQL Server you can use the dense_rank and additional windowing functions to get all rows AND columns with duplicated values on specified columns. Here is an example...

with t as (
    select col1 = \'a\', col2 = \'b\', col3 = \'c\', other = \'r1\' union all
    select col1 = \'c\', col2 = \'b\', col3 = \'a\', other = \'r2\' union all
    select col1 = \'a\', col2 = \'b\', col3 = \'c\', other = \'r3\' union all
    select col1 = \'a\', col2 = \'b\', col3 = \'c\', other = \'r4\' union all
    select col1 = \'c\', col2 = \'b\', col3 = \'a\', other = \'r5\' union all
    select col1 = \'a\', col2 = \'a\', col3 = \'a\', other = \'r6\'
), tdr as (
    select 
        *, 
        total_dr_rows = count(*) over(partition by dr)
    from (
        select 
            *, 
            dr = dense_rank() over(order by col1, col2, col3),
            dr_rn = row_number() over(partition by col1, col2, col3 order by other)
        from 
            t
    ) x
)

select * from tdr where total_dr_rows > 1

This is taking a row count for each distinct combination of col1, col2, and col3.

回答9:

SELECT *
FROM tblname
GROUP BY duplicate_values
ORDER BY ex.VISITED_ON DESC
LIMIT 0 , 30

in ORDER BY i have just put example here, you can also add ID field in this

回答10:

Add GROUP BY to field you want to check for duplicates your query may look like

SELECT field1, field2, field3, ......   FROM table GROUP BY field1

field1 will be checked to exclude duplicate records

or you may query like

SELECT *  FROM table GROUP BY field1

duplicate records of field1 are excluded from SELECT

回答11:

I would suggest using

SELECT  * from table where field1 in 
(
  select distinct field1 from table
)

this way if you have the same value in field1 across multiple rows, all the records will be returned.

回答12:

Just include all of your fields in the GROUP BY clause.

回答13:

It can be done by inner query

$query = \"SELECT * 
            FROM (SELECT field
                FROM table
                ORDER BY id DESC) as rows               
            GROUP BY field\";

回答14:

SELECT * from table where field in (SELECT distinct field from table)

回答15:

SELECT DISTINCT FIELD1, FIELD2, FIELD3 FROM TABLE1 works if the values of all three columns are unique in the table.

If, for example, you have multiple identical values for first name, but the last name and other information in the selected columns is different, the record will be included in the result set.