axiom-sqlitedata-ref

# SQLiteData Advanced Reference

## Overview

Advanced query patterns and schema composition techniques for [SQLiteData](https://github.com/pointfreeco/sqlite-data) by Point-Free. Built on [GRDB](https://github.com/groue/GRDB.swift) and [StructuredQueries](https://github.com/pointfreeco/swift-structured-queries).

**For core patterns** (CRUD, CloudKit setup, @Table basics), see the `axiom-sqlitedata` discipline skill.

**This reference covers** advanced querying, schema composition, views, and custom aggregates.

**Requires** iOS 17+, Swift 6 strict concurrency
**Framework** SQLiteData 1.4+

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## Column Groups and Schema Composition

SQLiteData provides powerful tools for composing schema types, enabling reuse, better organization, and single-table inheritance patterns.

### Column Groups

Group related columns into reusable types with `@Selection`:

```swift
// Define a reusable column group
@Selection
struct Timestamps {
    let createdAt: Date
    let updatedAt: Date?
}

// Use in multiple tables
@Table
nonisolated struct RemindersList: Identifiable {
    let id: UUID
    var title = ""
    let timestamps: Timestamps  // Embedded column group
}

@Table
nonisolated struct Reminder: Identifiable {
    let id: UUID
    var title = ""
    var isCompleted = false
    let timestamps: Timestamps  // Same group, reused
}
```

**Important:** SQLite has no concept of grouped columns. Flatten all groupings in your CREATE TABLE:

```sql
CREATE TABLE "remindersLists" (
    "id" TEXT PRIMARY KEY NOT NULL DEFAULT (uuid()),
    "title" TEXT NOT NULL DEFAULT '',
    "createdAt" TEXT NOT NULL,
    "updatedAt" TEXT
) STRICT
```

#### Querying Column Groups

Access fields inside groups with dot syntax:

```swift
// Query a field inside the group
RemindersList
    .where { $0.timestamps.createdAt <= cutoffDate }
    .fetchAll(db)

// Compare entire group (flattens to tuple in SQL)
RemindersList
    .where {
        $0.timestamps <= Timestamps(createdAt: date1, updatedAt: date2)