Postgres Count without groupby (using jpa)

Question

I have been using mariaDB, and now I am moving to postgres. i have a query in JPA

select count(<primarykeycolumn>) from tablename where condition

But Postgres gives an error saying a group by clause is needed.

I don't want to add a group by, the code is up and running with mariaDB.

Below is the error:

Internal Exception: org.postgresql.util.PSQLException: ERROR: column "app_movie_summary.creationdate" must appear in the GROUP BY clause or be used in an aggregate function
  Position: 55
Error Code: 0
Call: SELECT COUNT(id) FROM mytable ORDER BY CREATIONDATE DESC
Query: ReportQuery(name="App_Movie_Summary.findAllByCreationDateCount" referenceClass=App_Movie_Summary sql="SELECT COUNT(Id) FROM mytable ORDER BY CREATIONDATE DESC")
    at org.eclipse.persistence.exceptions.DatabaseException.sqlException(DatabaseException.java:340)
    at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.basicExecuteCall(DatabaseAccessor.java:684)
    at org.eclipse.persistence.internal.databaseaccess.DatabaseAccessor.executeCall(DatabaseAccessor.java:560)
    at org.eclipse.persistence.internal.sessions.AbstractSession.basicExecuteCall(AbstractSession.java:2055)
    at org.eclipse.persistence.sessions.server.ServerSession.executeCall(ServerSession.java:570)
    at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeCall(DatasourceCallQueryMechanism.java:242)
    at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeCall(DatasourceCallQueryMechanism.java:228)
    at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.executeSelectCall(DatasourceCallQueryMechanism.java:299)
    at org.eclipse.persistence.internal.queries.DatasourceCallQueryMechanism.selectAllRows(DatasourceCallQueryMechanism.java:694)
    at org.eclipse.persistence.internal.queries.ExpressionQueryMechanism.selectAllRowsFromTable(ExpressionQueryMechanism.java:2740)
    at org.eclipse.persistence.internal.queries.ExpressionQueryMechanism.selectAllReportQueryRows(ExpressionQueryMechanism.java:2677)
    at org.eclipse.persistence.queries.ReportQuery.executeDatabaseQuery(ReportQuery.java:852)
    at org.eclipse.persistence.queries.DatabaseQuery.execute(DatabaseQuery.java:904)
    at org.eclipse.persistence.queries.ObjectLevelReadQuery.execute(ObjectLevelReadQuery.java:1134)
    at org.eclipse.persistence.queries.ReadAllQuery.execute(ReadAllQuery.java:460)
    at org.eclipse.persistence.queries.ObjectLevelReadQuery.executeInUnitOfWork(ObjectLevelReadQuery.java:1222)
    at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.internalExecuteQuery(UnitOfWorkImpl.java:2896)
    at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1857)
    at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1839)
    at org.eclipse.persistence.internal.sessions.AbstractSession.executeQuery(AbstractSession.java:1804)
    at org.eclipse.persistence.internal.jpa.QueryImpl.executeReadQuery(QueryImpl.java:258)
    at org.eclipse.persistence.internal.jpa.QueryImpl.getSingleResult(QueryImpl.java:521)
    at org.eclipse.persistence.internal.jpa.EJBQueryImpl.getSingleResult(EJBQueryImpl.java:400)

Answer 1

In PostgreSQL, you cannot mix aggregate functions (like count ) with unaggregated columns ( creationdates ) unless you GROUP BY the unaggregated columns. In this case, it doesn't look like you need to group after all because you have no need to ORDER BY creationdate . Order is irrelevant to count(*) . This query is perfectly valid:

SELECT count(*) FROM mytable

If you do want creationdate included, you must GROUP BY it. When you do that, ORDER BY also makes sense again:

SELECT creationdate, count(*) from mytable
GROUP BY creationdate
ORDER BY creationdate