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Provides the Query DSL.

Queries are used to retrieve and manipulate data in a repository
(see `Ecto.Repo`). Although this module provides a complete API,
supporting expressions like `where/3`, `select/3` and so forth,
most of the times developers need to import only the `from/2`

    # Imports only from/1 and from/2 from Ecto.Query
    import Ecto.Query, only: [from: 2]

    # Create a query
    query = from w in Weather,
          where: w.prcp > 0,

    # Send the query to the repository

## Composition

Ecto queries are composable. For example, the query above can
actually be defined in two parts:

    # Create a query
    query = from w in Weather, where: w.prcp > 0

    # Extend the query
    query = from w in query, select:

Keep in mind though the variable names used on the left-hand
side of `in` are just a convenience, they are not taken into
account in the query generation.

Any value can be used on the right-side of `in` as long as it
implements the `Ecto.Queryable` protocol.

## Query expressions

Ecto allows a limitted set of expressions to be used inside queries:

  * Comparison operators: `==`, `!=`, `<=`, `>=`, `<`, `>`
  * Boolean operators: `and`, `or`, `not`
  * Inclusion operator: `in/2`
  * Search functions: `like/2` and `ilike/2`
  * Null check functions: `is_nil/1`
  * Aggregates: `count/1`, `avg/1`, `sum/1`, `min/1`, `max/1`

Futhermore, Ecto allows the following literals inside queries:

  * Integers: `1`, `2`, `3`
  * Floats: `1.0`, `2.0`, `3.0`
  * Booleans: `true`, `false`
  * Binaries: `<<1, 2, 3>>`
  * Strings: `"foo bar"`, `~s(this is a string)`
  * Arrays: `[1, 2, 3]`, `~w(interpolate words)`
  * UUIDs: `uuid("0123456789abcdef")`

External values and Elixir expressions can be injected into a query
expression with `^`:

    def with_minimum(age, height_ft) do
        from u in User,
      where: u.age > ^age and u.height > ^(height_ft * 3.28)

    with_minimum(18, 5.0)

Finally, Ecto provides two conveniences for dynamically generating
queries. The first is the `field/2` function which allows developers
to dynamically choose a field to query:

    def at_least_four(doors_or_tires) do
        from c in Car,
      where: field(u, ^doors_or_tires) >= 4

In the example above, both `at_least_four(:doors)` and `at_least_four(:tires)`
would be valid calls as the field is dynamically inserted.

The other convenience is called fragments which allows developers to send
any expression to the database via the `fragment(...)` function:

    def unpublished_by_title(title)
      from p in Post,
        where: is_nil(p.published_at) and
               fragment("downcase(?) == ?", p.title, ^title)

Fragments are sent directly to the database while also allowing field names
like `p.title` and values like `^title` to be interpolated.

## Casting

Ecto is able to cast interpolated values in queries:

    age = "1"
    Repo.all(from u in User, where: u.age > ^age)

The example above will work even if `age` is tagged as an :integer
in the User module because Ecto is able to cast values. In case a
value cannot be cast, `Ecto.CastError` is raised.

It is important to keep in mind that Ecto cannot cast nil values in
queries. Passing nil automatically causes the query to fail.

## Query expansion

In all examples so far, we have used the **keywords query syntax** to create
a query. Our first example:

    import Ecto.Query

       from w in Weather,
     where: w.prcp > 0,

Simply expands to the following **query expressions**:

    from(w in Weather) |> where([w], w.prcp > 0) |> select([w],

Which then expands to:

    select(where(from(w in Weather), [w], w.prcp > 0), [w],

This module documents each of those macros, providing examples both
in the keywords query and in the query expression formats.
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