# Queries

### Overview

Midio expressions support a specialized query expression syntax for working with lists of data in a clear, readable, and declarative way.

A query expression is composed of a sequence of clauses that are evaluated top to bottom:

* `from` introduces a variable that iterates over a collection
* `where` filters elements based on a condition (optional)
* `join` combines elements from multiple collections based on a predicate (optional)
* `select` defines the final shape of the result

At minimum, a query must contain a `from` clause and a `select` clause. Additional clauses refine or enrich the data as needed.

***

### Basic Queries (`from` / `select`)

#### Example 1: Transforming values

Convert each number in a list into a new value.

```
from n in [1, 2, 3, 4]
select n * 2
```

***

#### Example 2: Selecting a field from objects

Extract a single field from each object.

```
from u in [
  { email: "a@example.com" },
  { email: "b@example.com" }
]
select u.email
```

***

#### Example 3: Constructing new objects

Build a new object for each element in the collection.

```
from p in [
  { name: "Pen", price: 10 },
  { name: "Notebook", price: 25 }
]
select {
  name: p.name,
  price: p.price
}
```

***

### Filtering with `where`

#### Example 4: Filtering values

Keep only values that satisfy a condition.

```
from n in [5, 12, 30]
where n > 10
select n
```

***

#### Example 5: Filtering objects

Filter objects before selecting a field.

```
from u in [
  { email: "a@example.com", isActive: true },
  { email: "b@example.com", isActive: false }
]
where u.isActive
select u.email
```

***

#### Example 6: Filtering and transforming

Filter objects and return a transformed result.

```
from o in [
  { id: 1, total: 50 },
  { id: 2, total: 200 }
]
where o.total > 100
select {
  id: o.id,
  total: o.total
}
```

***

### Joining Collections (`join`)

#### Example 7: Simple join

**Description:** Combine related data from two collections.

```
from u in [ { id: 1, name: "Alice" } ]
join o in [ { userId: 1, total: 100 } ] 
  on u.id == o.userId
select {
  user: u.name,
  total: o.total
}
```

***

#### Example 8: Join with filtering

Join data and filter the combined result.

```
from u in [ { id: 1, name: "Alice" } ]
join o in [ { userId: 1, total: 1000 } ] 
  on u.id == o.userId
where o.total > 500
select {
  user: u.name,
  total: o.total
}
```

***

#### Example 9: Join with conditions on both sides

Use fields from both collections in the filter.

```
from u in [ { id: 1, name: "Alice", isActive: true } ]
join o in [ { userId: 1, id: 10, total: 1000 } ] 
  on u.id == o.userId
where u.isActive && o.total > 500
select {
  user: u.name,
  orderId: o.id
}
```


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