Select where count of one field is greater than one

Question

I want to do something like this:

SELECT * 
  FROM db.table 
 WHERE COUNT(someField) > 1

How can I achieve this in MySql?

Answer 1

Use the HAVING, not WHERE clause, for aggregate result comparison.

Taking the query at face value:

SELECT * 
  FROM db.table 
HAVING COUNT(someField) > 1

Ideally, there should be a GROUP BY defined for proper valuation in the HAVING clause, but MySQL does allow hidden columns from the GROUP BY...

Is this in preparation for a unique constraint on someField? Looks like it should be...

Answer 2

It should also be mentioned that the "pk" should be a key field. The self-join

SELECT t1.* FROM db.table t1
JOIN db.table t2 ON t1.someField = t2.someField AND t1.pk != t2.pk 

by Bill Karwin give you all the records that are duplicates which is what I wanted. Because some have more than two, you can get the same record more than once. I wrote all to another table with the same fields to get rid of the same records by key fields suppression. I tried

SELECT * FROM db.table HAVING COUNT(someField) > 1

above first. The data returned from it give only one of the duplicates, less than 1/2 of what this gives you but the count is good if that is all you want.

Answer 3

As OMG Ponies stated, the having clause is what you are after. However, if you were hoping that you would get discrete rows instead of a summary (the "having" creates a summary) - it cannot be done in a single statement. You must use two statements in that case.

Answer 4

I give an example up on Group By between two table in Sql:

Select cn.name,ct.name,count(ct.id) totalcity from city ct left join country cn on ct.countryid = cn.id Group By cn.name,ct.name Having totalcity > 2

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Answer 5

One way

SELECT t1.* 
FROM db.table t1
WHERE exists 
      (SELECT *
      FROM db.table t2 
      where t1.pk != t2.pk 
      and t1.someField = t2.someField)

Answer 6

Here you go:

SELECT Field1, COUNT(Field1)
  FROM Table1 
 GROUP BY Field1
HAVING COUNT(Field1) > 1
ORDER BY Field1 desc