@tanstack/db/skills/db-core/live-queries/SKILL.md

A reactive client store for building super fast apps on sync

---
name: db-core/live-queries
description: >
  Query builder fluent API: from, where, join, leftJoin, rightJoin, innerJoin,
  fullJoin, select, fn.select, groupBy, having, orderBy, limit, offset, distinct,
  findOne. Operators: eq, gt, gte, lt, lte, like, ilike, inArray, isNull,
  isUndefined, and, or, not. Aggregates: count, sum, avg, min, max. String
  functions: upper, lower, length, concat. Utility: coalesce, caseWhen. Math:
  add, subtract, multiply, divide.
  $selected namespace. createLiveQueryCollection. Derived collections. Predicate push-down.
  Incremental view maintenance via differential dataflow (d2ts). Virtual
  properties ($synced, $origin, $key, $collectionId). Includes subqueries
  for hierarchical data. Collection, toArray, materialize, and
  concat(toArray(...)) include modes.
  queryOnce for one-shot queries. createEffect for reactive side effects
  (onEnter, onUpdate, onExit, onBatch).
type: sub-skill
library: db
library_version: '0.6.17'
sources:
  - 'TanStack/db:docs/guides/live-queries.md'
  - 'TanStack/db:packages/db/src/query/builder/index.ts'
  - 'TanStack/db:packages/db/src/query/compiler/index.ts'
---

# Live Queries

> This skill builds on db-core.

TanStack DB live queries use a SQL-like fluent query builder to create **reactive derived collections** that automatically update when underlying data changes. The query engine compiles queries into incremental view maintenance (IVM) pipelines using differential dataflow (d2ts), so only deltas are recomputed.

All operators, string functions, math functions, and aggregates are incrementally maintained. Prefer them over equivalent JS code.

## Setup

Minimal example using the core API (no framework hooks):

```ts
import {
  createCollection,
  createLiveQueryCollection,
  liveQueryCollectionOptions,
  eq,
} from '@tanstack/db'

// Assume usersCollection is already created via createCollection(...)

// Option 1: createLiveQueryCollection shorthand
const activeUsers = createLiveQueryCollection((q) =>
  q
    .from({ user: usersCollection })
    .where(({ user }) => eq(user.active, true))
    .select(({ user }) => ({
      id: user.id,
      name: user.name,
      email: user.email,
    })),
)

// Option 2: full options via liveQueryCollectionOptions
const activeUsers2 = createCollection(
  liveQueryCollectionOptions({
    query: (q) =>
      q
        .from({ user: usersCollection })
        .where(({ user }) => eq(user.active, true))
        .select(({ user }) => ({
          id: user.id,
          name: user.name,
        })),
    getKey: (user) => user.id,
  }),
)

// The result is a live collection -- iterate, subscribe, or use as source
for (const user of activeUsers) {
  console.log(user.name)
}
```

## Core Patterns

### 1. Filtering with where + operators

Chain `.where()` calls (ANDed together) using expression operators. Use `and()`, `or()`, `not()` for complex logic.

```ts
import { eq, gt, or, and, not, inArray, like } from '@tanstack/db'

const results = createLiveQueryCollection((q) =>
  q
    .from({ user: usersCollection })
    .where(({ user }) => eq(user.active, true))
    .where(({ user }) =>
      and(
        gt(user.age, 18),
        or(eq(user.role, 'admin'), eq(user.role, 'moderator')),
        not(inArray(user.id, bannedIds)),
      ),
    ),
)
```

Boolean column references work directly:

```ts
.where(({ user }) => user.active)        // bare boolean ref
.where(({ user }) => not(user.suspended)) // negated boolean ref
```

Comparisons follow PostgreSQL semantics. Comparisons involving `null` or
`undefined` are unknown and do not match; use `isNull()` or `isUndefined()`.
`NaN` (and an invalid `Date`) equals itself and sorts after every other
non-null value.

### 2. Joining two collections

Join conditions **must** use `eq()` (equality only -- IVM constraint). Default join type is `left`. Convenience methods: `leftJoin`, `rightJoin`, `innerJoin`, `fullJoin`.

```ts
import { eq } from '@tanstack/db'

const userPosts = createLiveQueryCollection((q) =>
  q
    .from({ user: usersCollection })
    .innerJoin({ post: postsCollection }, ({ user, post }) =>
      eq(user.id, post.userId),
    )
    .select(({ user, post }) => ({
      userName: user.name,
      postTitle: post.title,
    })),
)
```

Multiple joins:

```ts
q.from({ user: usersCollection })
  .join({ post: postsCollection }, ({ user, post }) => eq(user.id, post.userId))
  .join({ comment: commentsCollection }, ({ post, comment }) =>
    eq(post.id, comment.postId),
  )
```

### 3. Aggregation with groupBy + having

Use `groupBy` to group rows, then aggregate in `select`. Filter groups with `having`. The `$selected` namespace lets `having` and `orderBy` reference fields defined in `select`.

```ts
import { count, sum, gt } from '@tanstack/db'

const topCustomers = createLiveQueryCollection((q) =>
  q
    .from({ order: ordersCollection })
    .groupBy(({ order }) => order.customerId)
    .select(({ order }) => ({
      customerId: order.customerId,
      totalSpent: sum(order.amount),
      orderCount: count(order.id),
    }))
    .having(({ $selected }) => gt($selected.totalSpent, 1000))
    .orderBy(({ $selected }) => $selected.totalSpent, 'desc')
    .limit(10),
)
```

Without `groupBy`, aggregates in `select` treat the entire collection as one group:

```ts
const stats = createLiveQueryCollection((q) =>
  q.from({ user: usersCollection }).select(({ user }) => ({
    totalUsers: count(user.id),
    avgAge: avg(user.age),
  })),
)
```

### 4. Standalone derived collection with createLiveQueryCollection

Derived collections are themselves collections. Use one as a source for another query to cache intermediate results:

```ts
// Base derived collection
const activeUsers = createLiveQueryCollection((q) =>
  q.from({ user: usersCollection }).where(({ user }) => eq(user.active, true)),
)

// Second query uses the derived collection as its source
const activeUserPosts = createLiveQueryCollection((q) =>
  q
    .from({ user: activeUsers })
    .join({ post: postsCollection }, ({ user, post }) =>
      eq(user.id, post.userId),
    )
    .select(({ user, post }) => ({
      userName: user.name,
      postTitle: post.title,
    })),
)
```

Create derived collections once at module scope and reuse them. Do not recreate on every render or navigation.

Live query collections default to `gcTime: 5_000`. An explicit `gcTime: 0` is
preserved and disables garbage collection for that derived collection.

## Virtual Properties

Live query results include computed, read-only virtual properties on every row:

- `$synced`: `true` when no pending local optimistic write affects the row;
  `false` while one does. This is local mutation status, not proof that a
  backend uploaded, confirmed, or read back the row.
- `$origin`: `"local"` if the last confirmed change came from this client, otherwise `"remote"`.
- `$key`: the row key for the result.
- `$collectionId`: the source collection ID.

These props are added automatically and can be used in `where`, `select`, and `orderBy` clauses. Do not persist them back to storage.

## Includes (Subqueries in Select)

Embed a correlated subquery inside `select()` to produce hierarchical (nested)
data. The subquery must contain a `where` with an `eq()` that correlates a
parent field with a child field.

### Collection includes (default)

Return a child `Collection` on each parent row:

```ts
import { eq, createLiveQueryCollection } from '@tanstack/db'

const projectsWithIssues = createLiveQueryCollection((q) =>
  q.from({ p: projectsCollection }).select(({ p }) => ({
    id: p.id,
    name: p.name,
    issues: q
      .from({ i: issuesCollection })
      .where(({ i }) => eq(i.projectId, p.id))
      .select(({ i }) => ({
        id: i.id,
        title: i.title,
      })),
  })),
)

// Each row's `issues` is a live Collection
for (const project of projectsWithIssues) {
  console.log(project.name, project.issues.toArray)
}
```

### Array includes with toArray()

Wrap the subquery in `toArray()` to get a plain array instead of a Collection:

```ts
import { eq, toArray, createLiveQueryCollection } from '@tanstack/db'

const messagesWithParts = createLiveQueryCollection((q) =>
  q.from({ m: messagesCollection }).select(({ m }) => ({
    id: m.id,
    contentParts: toArray(
      q
        .from({ c: chunksCollection })
        .where(({ c }) => eq(c.messageId, m.id))
        .orderBy(({ c }) => c.timestamp)
        .select(({ c }) => c.text),
    ),
  })),
)
// row.contentParts is string[]
```

### Plain values with materialize()

Use `materialize()` when the parent row should hold a plain snapshot rather
than a child collection:

```ts
import { eq, materialize, createLiveQueryCollection } from '@tanstack/db'

const issuesWithProject = createLiveQueryCollection((q) =>
  q.from({ issue: issuesCollection }).select(({ issue }) => ({
    ...issue,
    project: materialize(
      q
        .from({ project: projectsCollection })
        .where(({ project }) => eq(project.id, issue.projectId))
        .findOne(),
    ),
  })),
)
// row.project is Project | undefined
```

For a multi-row subquery, `materialize()` returns `Array<T>` like `toArray()`.
For a subquery ending in `findOne()`, it returns `T | undefined`. In both cases,
the parent row is re-emitted when the child result changes.

### Concatenated scalar with concat(toArray())

Wrap `toArray()` in `concat()` to join the scalar results into a single string:

```ts
import { eq, toArray, concat, createLiveQueryCollection } from '@tanstack/db'

const messagesWithContent = createLiveQueryCollection((q) =>
  q.from({ m: messagesCollection }).select(({ m }) => ({
    id: m.id,
    content: concat(
      toArray(
        q
          .from({ c: chunksCollection })
          .where(({ c }) => eq(c.messageId, m.id))
          .orderBy(({ c }) => c.timestamp)
          .select(({ c }) => c.text),
      ),
    ),
  })),
)
// row.content is a single concatenated string
```

### Includes rules

- The subquery **must** have a `where` clause with an `eq()` correlating a parent alias with a child alias. The library extracts this automatically as the join condition.
- `toArray()` works with both scalar selects (e.g., `select(({ c }) => c.text)` → `string[]`) and object selects (e.g., `select(({ c }) => ({ id: c.id, title: c.title }))` → `Array<{id, title}>`).
- `materialize()` returns an array, or one value for a `findOne()` subquery.
  Like `toArray()`, it must be a top-level value in `select()` and cannot be
  nested inside `coalesce()`, `eq()`, or another expression.
- `concat(toArray())` requires a **scalar** `select` to concatenate into a string.
- Collection includes (bare subquery) require an **object** `select`.
- Includes subqueries are compiled into the same incremental pipeline as the parent query -- they are not separate live queries.

## One-Shot Queries with queryOnce

For non-reactive, one-time snapshots use `queryOnce`. It creates a live query collection, preloads it, extracts the results, and cleans up automatically.

```ts
import { eq, queryOnce } from '@tanstack/db'

const activeUsers = await queryOnce((q) =>
  q
    .from({ user: usersCollection })
    .where(({ user }) => eq(user.active, true))
    .select(({ user }) => ({ id: user.id, name: user.name })),
)

// With findOne — resolves to T | undefined
const user = await queryOnce((q) =>
  q
    .from({ user: usersCollection })
    .where(({ user }) => eq(user.id, userId))
    .findOne(),
)
```

Use `queryOnce` for scripts, loaders, data export, tests, or AI/LLM context building. For UI bindings and reactive updates, use live queries instead.

## Reactive Effects (createEffect)

Reactive effects respond to query result _changes_ without materializing the full result set. Effects fire callbacks when rows enter, exit, or update within a query result — like a database trigger on an arbitrary live query.

```ts
import { createEffect, eq } from '@tanstack/db'

const effect = createEffect({
  query: (q) =>
    q
      .from({ msg: messagesCollection })
      .where(({ msg }) => eq(msg.role, 'user')),
  skipInitial: true,
  onEnter: async (event, ctx) => {
    await processNewMessage(event.value, { signal: ctx.signal })
  },
  onExit: (event) => {
    console.log('Message left result set:', event.key)
  },
  onError: (error, event) => {
    console.error(`Failed to process ${event.key}:`, error)
  },
})

// Dispose when no longer needed
await effect.dispose()
```

| Use case                        | Approach                                              |
| ------------------------------- | ----------------------------------------------------- |
| Display query results in UI     | Live query collection + `useLiveQuery`                |
| React to changes (side effects) | `createEffect` with `onEnter` / `onUpdate` / `onExit` |
| Inspect full batch of changes   | `createEffect` with `onBatch`                         |

Key options: `id` (optional), `query`, `skipInitial` (skip existing rows on init), `onEnter`, `onUpdate`, `onExit`, `onBatch`, `onError`, `onSourceError`. The `ctx.signal` aborts when the effect is disposed.

## Common Mistakes

### CRITICAL: Using === instead of eq()

JavaScript `===` in a where callback returns a boolean primitive, not an expression object. Throws `InvalidWhereExpressionError`.

```ts
// WRONG
q.from({ user: usersCollection }).where(({ user }) => user.active === true)

// CORRECT
q.from({ user: usersCollection }).where(({ user }) => eq(user.active, true))
```

### CRITICAL: Filtering in JS instead of query operators

JS `.filter()` / `.map()` on the result array throws away incremental maintenance -- the JS code re-runs from scratch on every change.

```ts
// WRONG -- re-runs filter on every change
const { data } = useLiveQuery((q) => q.from({ todos: todosCollection }))
const active = data.filter((t) => t.completed === false)

// CORRECT -- incrementally maintained
const { data } = useLiveQuery((q) =>
  q
    .from({ todos: todosCollection })
    .where(({ todos }) => eq(todos.completed, false)),
)
```

### HIGH: Not using the full operator set

The library provides string functions (`upper`, `lower`, `length`, `concat`),
math (`add`, `subtract`, `multiply`, `divide`), utility functions (`coalesce`,
`caseWhen`), and aggregates (`count`, `sum`, `avg`, `min`, `max`). All are
incrementally maintained. Prefer them over JS equivalents.

```ts
// WRONG
.fn.select((row) => ({
  name: row.user.name.toUpperCase(),
  total: row.order.price + row.order.tax,
}))

// CORRECT
.select(({ user, order }) => ({
  name: upper(user.name),
  total: add(order.price, order.tax),
  displayName: coalesce(user.displayName, user.name, 'Unknown'),
}))
```

Math expressions also work in `orderBy()`. When a computed expression is used
with `limit()`, lazy-loading optimization is skipped and all matching rows load
before sorting. Literal values such as `Date.now()` are captured when the query
is created; recreate the query when the value must advance.

### HIGH: Missing conditional expression helpers

Use `coalesce()` for null/undefined fallbacks and `caseWhen()` for conditional
computed fields. JavaScript operators like `||` or ternaries do not build query
expressions inside standard `.select()` callbacks.

```ts
// WRONG -- document.title is a query ref, not a runtime string
.select(({ document }) => ({
  displayTitle: document.title || 'Untitled document',
}))

// CORRECT -- fallback for null/undefined
.select(({ document }) => ({
  displayTitle: coalesce(document.title, 'Untitled document'),
}))

// CORRECT -- fallback for null/undefined and empty string
.select(({ document }) => ({
  displayTitle: caseWhen(
    eq(coalesce(document.title, ''), ''),
    'Untitled document',
    document.title,
  ),
}))
```

Use `fn.select()` only when you genuinely need arbitrary JavaScript; it cannot
be optimized like expression-based `.select()`.

### HIGH: .distinct() without .select()

`distinct()` deduplicates by the selected columns. Without `select()`, throws `DistinctRequiresSelectError`.

```ts
// WRONG
q.from({ user: usersCollection }).distinct()

// CORRECT
q.from({ user: usersCollection })
  .select(({ user }) => ({ country: user.country }))
  .distinct()
```

### HIGH: .having() without .groupBy()

`having` filters aggregated groups. Without `groupBy`, there are no groups. Throws `HavingRequiresGroupByError`.

```ts
// WRONG
q.from({ order: ordersCollection }).having(({ order }) =>
  gt(count(order.id), 5),
)

// CORRECT
q.from({ order: ordersCollection })
  .groupBy(({ order }) => order.customerId)
  .having(({ order }) => gt(count(order.id), 5))
```

### HIGH: .limit() / .offset() without .orderBy()

Without deterministic ordering, limit/offset results are non-deterministic and cannot be incrementally maintained. Throws `LimitOffsetRequireOrderByError`.

```ts
// WRONG
q.from({ user: usersCollection }).limit(10)

// CORRECT
q.from({ user: usersCollection })
  .orderBy(({ user }) => user.name)
  .limit(10)
```

### HIGH: Join condition using non-eq() operator

The differential dataflow join operator only supports equality joins. Using `gt()`, `like()`, etc. throws `JoinConditionMustBeEqualityError`.

```ts
// WRONG
q.from({ user: usersCollection }).join(
  { post: postsCollection },
  ({ user, post }) => gt(user.id, post.userId),
)

// CORRECT
q.from({ user: usersCollection }).join(
  { post: postsCollection },
  ({ user, post }) => eq(user.id, post.userId),
)
```

### MEDIUM: Passing source directly instead of {alias: collection}

`from()` and `join()` require sources wrapped as `{alias: collection}`. Passing the collection directly throws `InvalidSourceTypeError`.

```ts
// WRONG
q.from(usersCollection)

// CORRECT
q.from({ users: usersCollection })
```

### MEDIUM: Using unsafe select alias paths

Select alias path segments named `__proto__`, `prototype`, or `constructor`
throw `UnsafeAliasPathError`. Use ordinary data-field names; do not suppress
this prototype-pollution guard.

## Tension: Query expressiveness vs. IVM constraints

The query builder looks like SQL but has constraints that SQL does not:

- **Equality joins only** -- `eq()` is the only allowed join condition operator.
- **orderBy required for limit/offset** -- non-deterministic pagination cannot be incrementally maintained.
- **distinct requires select** -- deduplication needs an explicit projection.
- **fn.select() cannot be used with groupBy()** -- the compiler must statically analyze select to discover aggregate functions.

These constraints exist because the underlying d2ts differential dataflow engine requires them for correct incremental view maintenance.

See also: react-db/SKILL.md for React hooks (`useLiveQuery`, `useLiveSuspenseQuery`, `useLiveInfiniteQuery`).

## References

- [Query Operators Reference](./references/operators.md) -- full signatures and examples for all operators, functions, and aggregates.