[Home](./index.md) > [@firebase/database](./database.md) > [QueryConstraint](./database.queryconstraint.md) ## QueryConstraint class A `QueryConstraint` is used to narrow the set of documents returned by a Database query. `QueryConstraint`s are created by invoking [endAt()](./database.endat.md), [endBefore()](./database.endbefore.md), [startAt()](./database.startat.md), [startAfter()](./database.startafter.md), [limitToFirst()](./database.limittofirst.md), [limitToLast()](./database.limittolast.md), [orderByChild()](./database.orderbychild.md), [orderByChild()](./database.orderbychild.md), [orderByKey()](./database.orderbykey.md) , [orderByPriority()](./database.orderbypriority.md) , [orderByValue()](./database.orderbyvalue.md) or [equalTo()](./database.equalto.md) and can then be passed to [query()](./database.query.md) to create a new query instance that also contains this `QueryConstraint`. Signature: ```typescript export declare abstract class QueryConstraint ``` ## Properties | Property | Modifiers | Type | Description | | --- | --- | --- | --- | | [type](./database.queryconstraint.type.md) | | [QueryConstraintType](./database.queryconstrainttype.md) | The type of this query constraints |