Obtaining distinct rows from MySQL in Java

Question

I have bind my combobox with MySQL database using GUI in netbeans. I get all year in combobox. However i am not being able to obtain the distinct years in combobox.

The query that displays all year is :
query = "SELECT  p FROM Production p WHERE p.yearOfProduction = :yearOfProduction";

I modified the query as below to display the distinct yearOfProduction. However the query doesnot change the list in combobox.

query = "SELECT DISTINCT p FROM Production p WHERE p.yearOfProduction = :yearOfProduction ORDER BY yearOfProduction";

What should i do ??? this type of query is new to me.

The java autogenerated code is:

  /*
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     * To change this template file, choose Tools | Templates
     * and open the template in the editor.
     */

package my_ui;

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.io.Serializable;
import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Table;
import javax.persistence.Transient;

/**
 *
 * @author enjal
 */
@Entity
@Table(name = "production", catalog = "data2", schema = "")
@NamedQueries({
    @NamedQuery(name = "Production.findAll", query = "SELECT p FROM Production p"),
    @NamedQuery(name = "Production.findByProductionId", query = "SELECT p FROM Production p WHERE p.productionId = :productionId"),
    @NamedQuery(name = "Production.findByCropId", query = "SELECT p FROM Production p WHERE p.cropId = :cropId"),
    @NamedQuery(name = "Production.findByLocationId", query = "SELECT p FROM Production p WHERE p.locationId = :locationId"),
    @NamedQuery(name = "Production.findByArea", query = "SELECT p FROM Production p WHERE p.area = :area"),
    @NamedQuery(name = "Production.findByProductionAmount", query = "SELECT p FROM Production p WHERE p.productionAmount = :productionAmount"),
    @NamedQuery(name = "Production.findByYieldAmount", query = "SELECT p FROM Production p WHERE p.yieldAmount = :yieldAmount"),
    @NamedQuery(name = "Production.findByYearOfProduction", query = "SELECT  p FROM Production p WHERE p.yearOfProduction = :yearOfProduction" )})
//SELECT DISTINCT year_of_production FROM `production` ORDER BY year_of_production ASC
public class Production implements Serializable {
    @Transient
    private PropertyChangeSupport changeSupport = new PropertyChangeSupport(this);
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "production_id")
    private Integer productionId;
    @Basic(optional = false)
    @Column(name = "crop_id")
    private int cropId;
    @Basic(optional = false)
    @Column(name = "location_id")
    private String locationId;
    @Basic(optional = false)
    @Column(name = "area")
    private double area;
    @Basic(optional = false)
    @Column(name = "production_amount")
    private String productionAmount;
    @Basic(optional = false)
    @Column(name = "yield_amount")
    private double yieldAmount;
    @Basic(optional = false)
    @Column(name = "year_of_production")
    private String yearOfProduction;

    public Production() {
    }

    public Production(Integer productionId) {
        this.productionId = productionId;
    }

    public Production(Integer productionId, int cropId, String locationId, double area, String productionAmount, double yieldAmount, String yearOfProduction) {
        this.productionId = productionId;
        this.cropId = cropId;
        this.locationId = locationId;
        this.area = area;
        this.productionAmount = productionAmount;
        this.yieldAmount = yieldAmount;
        this.yearOfProduction = yearOfProduction;
    }

    public Integer getProductionId() {
        return productionId;
    }

    public void setProductionId(Integer productionId) {
        Integer oldProductionId = this.productionId;
        this.productionId = productionId;
        changeSupport.firePropertyChange("productionId", oldProductionId, productionId);
    }

    public int getCropId() {
        return cropId;
    }

    public void setCropId(int cropId) {
        int oldCropId = this.cropId;
        this.cropId = cropId;
        changeSupport.firePropertyChange("cropId", oldCropId, cropId);
    }

    public String getLocationId() {
        return locationId;
    }

    public void setLocationId(String locationId) {
        String oldLocationId = this.locationId;
        this.locationId = locationId;
        changeSupport.firePropertyChange("locationId", oldLocationId, locationId);
    }

    public double getArea() {
        return area;
    }

    public void setArea(double area) {
        double oldArea = this.area;
        this.area = area;
        changeSupport.firePropertyChange("area", oldArea, area);
    }

    public String getProductionAmount() {
        return productionAmount;
    }

    public void setProductionAmount(String productionAmount) {
        String oldProductionAmount = this.productionAmount;
        this.productionAmount = productionAmount;
        changeSupport.firePropertyChange("productionAmount", oldProductionAmount, productionAmount);
    }

    public double getYieldAmount() {
        return yieldAmount;
    }

    public void setYieldAmount(double yieldAmount) {
        double oldYieldAmount = this.yieldAmount;
        this.yieldAmount = yieldAmount;
        changeSupport.firePropertyChange("yieldAmount", oldYieldAmount, yieldAmount);
    }

    public String getYearOfProduction() {
        return yearOfProduction;
    }

    public void setYearOfProduction(String yearOfProduction) {
        String oldYearOfProduction = this.yearOfProduction;
        this.yearOfProduction = yearOfProduction;
        changeSupport.firePropertyChange("yearOfProduction", oldYearOfProduction, yearOfProduction);
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (productionId != null ? productionId.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Production)) {
            return false;
        }
        Production other = (Production) object;
        if ((this.productionId == null && other.productionId != null) || (this.productionId != null && !this.productionId.equals(other.productionId))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {



        return "" + yearOfProduction + "";
    }

    public void addPropertyChangeListener(PropertyChangeListener listener) {
        changeSupport.addPropertyChangeListener(listener);
    }

    public void removePropertyChangeListener(PropertyChangeListener listener) {
        changeSupport.removePropertyChangeListener(listener);
    }

}

Answer 1

select distinct p.yearOfProduction from Production p order by yearOfProduction **My opinion is try using group by then apply distinct keyboard **

Answer 2

Obtain distinct year by

 select distinct (p.yearOfProduction) FROM Production p 

Query should not require any year parameter. If you pass year, you will get only one year.