# Class ColumnSelect<T>

java.lang.Object
org.apache.cayenne.query.CacheableQuery
org.apache.cayenne.query.AbstractQuery
org.apache.cayenne.query.FluentSelect<T, ColumnSelect<T>>
org.apache.cayenne.query.ColumnSelect<T>

All Implemented Interfaces:
  `Serializable, Query, Select<T>`

public class ColumnSelect<T> extends FluentSelect<T, ColumnSelect<T>>

A helper builder for queries selecting individual properties based on the root object.

It can be used to select properties of the object itself, properties of related entities or some function calls (including aggregate functions).

Usage examples:

         // select list of names:
         List<String> names = ObjectSelect.columnQuery(Artist.class, Artist.ARTIST_NAME).select(context);

         // select count:
         long count = ObjectSelect.columnQuery(Artist.class, PropertyFactory.COUNT).selectOne();

         // select only required properties of an entity:
         List<Object[]> data = ObjectSelect.columnQuery(Artist.class, Artist.ARTIST_NAME, Artist.DATE_OF_BIRTH)
                                     .where(Artist.ARTIST_NAME.like("Picasso%))
                                     .select(context);

Note: this class can't be instantiated directly. Use `ObjectSelect`.

Since:
  4.0
See Also:
  - `ObjectSelect.columnQuery(Class, Property)`
  - Serialized Form

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `protected Collection<Property<?>>`
  `columns`
  `protected boolean`
  `singleColumn`

  ### Fields inherited from class FluentSelect

  `dbEntityName, distinct, entityName, entityType, having, havingExpressionIsActive, metaData, orderings, where`

  ### Fields inherited from class AbstractQuery

  `root`

  ### Fields inherited from class CacheableQuery

  `LOGGER`

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `ColumnSelect()`
  `protected`
  `ColumnSelect(ObjectSelect<T> select)`
  Copy constructor to convert ObjectSelect to ColumnSelect

- ## Method Summary

  Modifier and Type
  Method
  Description
  `<E> ColumnSelect<Object[]>`
  `aggregate(BaseProperty<E> property, String function, Class<E> type)`
  Select result of some function, that aggregates values.
  `ColumnSelect<Object[]>`
  `avg(NumericProperty<?> property)`
  Select average value of property
  `protected <E> ColumnSelect<E>`
  `column(Property<E> property)`
  `ColumnSelect<Object[]>`
  `columns(Collection<Property<?>> properties)`
  Add properties to select.
  `ColumnSelect<Object[]>`
  `columns(Property<?>... properties)`
  Add properties to select.
  `ColumnSelect<Object[]>`
  `count()`
  Shortcut for `columns(Property[])` columns}(Property.COUNT)
  `ColumnSelect<Object[]>`
  `count(BaseProperty<?> property)`
  Select COUNT(property)
  `protected org.apache.cayenne.query.ColumnSelectMetadata`
  `createMetadata()`
  `ColumnSelect<T>`
  `distinct()`
  Explicitly request distinct in query.
  `protected org.apache.cayenne.query.ColumnSelectMetadata`
  `getBaseMetaData()`
  `Collection<Property<?>>`
  `getColumns()`
  `QueryMetadata`
  `getMetaData(EntityResolver resolver)`
  Returns default select parameters.
  `ColumnSelect<T>`
  `having(String expressionString, Object... parameters)`
  Appends a having qualifier expression of this query, using provided expression String and an array of position parameters.
  `ColumnSelect<T>`
  `having(Expression expression)`
  Appends a having qualifier expression of this query.
  `<E> ColumnSelect<E>`
  `map(Function<T,E> mapper)`
  Maps result of this query by processing with a given function.
  `ColumnSelect<Object[]>`
  `max(ComparableProperty<?> property)`
  Select maximum value of property
  `ColumnSelect<Object[]>`
  `min(ComparableProperty<?> property)`
  Select minimum value of property
  `T`
  `selectFirst(ObjectContext context)`
  Selects a single object using provided context.
  `<E extends Number>
  ColumnSelect<Object[]>`
  `sum(NumericProperty<E> property)`
  Select sum of values
  `ColumnSelect<T>`
  `suppressDistinct()`
  Explicitly suppress distinct in query.

  ### Methods inherited from class FluentSelect

  `and, and, batchIterator, cacheGroup, cacheStrategy, cacheStrategy, castSelf, createSQLAction, dbEntityName, entityName, entityType, getActiveExpression, getDbEntityName, getEntityName, getEntityType, getHaving, getLimit, getOffset, getOrderings, getPageSize, getPrefetches, getQueryTimeout, getStatementFetchSize, getWhere, initWithProperties, isDistinct, isFetchingDataRows, iterate, iterator, joinExpression, limit, localCache, localCache, offset, or, or, orderBy, orderBy, orderBy, orderBy, pageSize, prefetch, prefetch, queryTimeout, resolveRoot, route, routePrefetches, select, selectOne, setActiveExpression, sharedCache, sharedCache, statementFetchSize, where, where`

  ### Methods inherited from class AbstractQuery

  `getRoot, setRoot, toString`

  ### Methods inherited from class CacheableQuery

  `getCacheGroup, getCacheStrategy, setCacheGroup, setCacheStrategy, useLocalCache, useLocalCache, useSharedCache, useSharedCache`

  ### Methods inherited from class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- ## Field Details

  - ### columns

    protected Collection<Property<?>> columns

  - ### singleColumn

    protected boolean singleColumn

- ## Constructor Details

  - ### ColumnSelect

    protected ColumnSelect()

  - ### ColumnSelect

    protected ColumnSelect(ObjectSelect<T> select)
    Copy constructor to convert ObjectSelect to ColumnSelect

- ## Method Details

  - ### createMetadata

    protected org.apache.cayenne.query.ColumnSelectMetadata createMetadata()
    Specified by:
      `createMetadata` in class `FluentSelect<T, ColumnSelect<T>>`

  - ### columns

    public ColumnSelect<Object[]> columns(Property<?>... properties)

    Add properties to select.

    Can be any properties that can be resolved against root entity type (root entity properties, function call expressions, properties of relationships, etc).

        List<Object[]> columns = ObjectSelect.columnQuery(Artist.class, Artist.ARTIST_NAME)
                                           .columns(Artist.ARTIST_SALARY, Artist.DATE_OF_BIRTH)
                                           .select(context);

    Parameters:
      `properties` - array of properties to select
    See Also:
      - `column(Property)`
      - `columns(Collection)`

  - ### columns

    public ColumnSelect<Object[]> columns(Collection<Property<?>> properties)

    Add properties to select.

    Can be any properties that can be resolved against root entity type (root entity properties, function call expressions, properties of relationships, etc).

    Parameters:
      `properties` - collection of properties, must contain at least one element
    See Also:
      - `columns(Property[])`

  - ### column

    protected <E> ColumnSelect<E> column(Property<E> property)

  - ### count

    public ColumnSelect<Object[]> count()

    Shortcut for `columns(Property[])` columns}(Property.COUNT)

  - ### count

    public ColumnSelect<Object[]> count(BaseProperty<?> property)

    Select COUNT(property)

    Can return different result than COUNT(*) as it will count only non null values

    See Also:
      - `count()`

  - ### min

    public ColumnSelect<Object[]> min(ComparableProperty<?> property)

    Select minimum value of property

    See Also:
      - `columns(Property[])`

  - ### max

    public ColumnSelect<Object[]> max(ComparableProperty<?> property)

    Select maximum value of property

    See Also:
      - `columns(Property[])`

  - ### avg

    public ColumnSelect<Object[]> avg(NumericProperty<?> property)

    Select average value of property

    See Also:
      - `columns(Property[])`

  - ### sum

    public <E extends Number> ColumnSelect<Object[]> sum(NumericProperty<E> property)

    Select sum of values

    See Also:
      - `columns(Property[])`

  - ### aggregate

    public <E> ColumnSelect<Object[]> aggregate(BaseProperty<E> property, String function, Class<E> type)

    Select result of some function, that aggregates values.

    Since:
      5.0
    See Also:
      - `columns(Property[])`

  - ### having

    public ColumnSelect<T> having(Expression expression)
    Appends a having qualifier expression of this query. An equivalent to `FluentSelect.and(Expression...)` that can be used a syntactic sugar.
    Returns:
      this object

  - ### having

    public ColumnSelect<T> having(String expressionString, Object... parameters)
    Appends a having qualifier expression of this query, using provided expression String and an array of position parameters. This is an equivalent to calling "and".
    Returns:
      this object

  - ### distinct

    public ColumnSelect<T> distinct()
    Explicitly request distinct in query.

  - ### suppressDistinct

    public ColumnSelect<T> suppressDistinct()
    Explicitly suppress distinct in query.

  - ### getColumns

    public Collection<Property<?>> getColumns()
    Overrides:
      `getColumns` in class `FluentSelect<T, ColumnSelect<T>>`

  - ### selectFirst

    public T selectFirst(ObjectContext context)
    Description copied from interface: `Select`
    Selects a single object using provided context. The query itself can match any number of objects, but will return only the first one. It returns null if no objects were matched.

    If it matched more than one object, the first object from the list is returned. This makes 'selectFirst' different from `Select.selectOne(ObjectContext)`, which would throw in this situation. 'selectFirst' is useful e.g. when the query is ordered and we only want to see the first object (e.g. "most recent news article"), etc.

    Selecting the first object via "Select.selectFirst(ObjectContext)" is more comprehensible than selecting via "ObjectContext.selectFirst(Select)", because implementations of "Select" set fetch size limit to one.

  - ### getMetaData

    public QueryMetadata getMetaData(EntityResolver resolver)
    Description copied from class: `AbstractQuery`
    Returns default select parameters.
    Specified by:
      `getMetaData` in interface `Query`
    Overrides:
      `getMetaData` in class `AbstractQuery`

  - ### getBaseMetaData

    protected org.apache.cayenne.query.ColumnSelectMetadata getBaseMetaData()
    Specified by:
      `getBaseMetaData` in class `CacheableQuery`

  - ### map

    public <E> ColumnSelect<E> map(Function<T,E> mapper)
    Maps result of this query by processing with a given function.
    Could be used to map plain Object[] to some domain-specific object.
    Note: this method could be called multiple time, result will be mapped by all functions in the call order.
    Type Parameters:
      `E` - new result type
    Parameters:
      `mapper` - function that maps result to the required type.
    Returns:
      this query with changed result type
    Since:
      4.2