Question

The question is for the example table structure:

mytable

fielda, fieldb
val1    valx
val2    valy
val3    valz
val1    vala

I want to select from mytable with a query:

fielda, fieldb
val1    valx
val1    vala

basically select * from table where fielda is duplicated.

then with a different query select from mytable

fielda, fieldb
val2    valy
val3    valz

val1 on fielda was encountered as a duplicate therefore it wasn’t selected and the rest were.

Answer 1

This query sorts the table which duplicated fieldA are on top of the list.

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT fieldA, COUNT(*) totalCount
            FROM tableName
            GROUP BY fieldA
        ) b ON a.fieldA = b.fieldA
ORDER BY b.totalCount DESC

• SQLFiddle Demo

This query selects duplicate records only.

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT fieldA, COUNT(*) totalCount
            FROM tableName
            GROUP BY fieldA
        ) b ON a.fieldA = b.fieldA
WHERE b.totalCount > 1
ORDER BY b.totalCount DESC

• SQLFiddle Demo

and this one, selects records which are not on the second query (above)

SELECT  a.*
FROM    tableName a
        INNER JOIN
        (
            SELECT fieldA, COUNT(*) totalCount
            FROM tableName
            GROUP BY fieldA
        ) b ON a.fieldA = b.fieldA
WHERE b.totalCount = 1
ORDER BY b.totalCount DESC

• SQLFiddle Demo