SELECT
The SELECT operation, once executed, retrieves documents from one or more collections and uses clauses like WHERE to specify conditions for filtering the documents to return.
DQL currently only supports the SELECT operation for document retrieval. With SELECT *, all responses result in full documents.
Expect more advanced SELECT capabilities in upcoming DQL releases. For more information, see DQL Roadmap Functionality.
The following syntax outlines the basic structure and optional clauses you can use within your SELECT statements to specify your conditions for retrieval.
For example, retrieve all documents in the cars collection WHERE the color property is set to the value 'blue' :
The following table provides an overview of the different clauses you can use to define specific conditions and calculations within your DQL SELECT statements, providing more granular control over your queries:
Clause | Description |
FROM | The required clause specifying the collection containing the documents for retrieval. (See FROM) |
WHERE | Applies filtering conditions to restrict the documents included in the result set. (See WHERE) |
ORDER BY | Specifies the sorting order of the result set based on one or more expressions. (See ORDER BY) |
LIMIT | Restricts the number of documents included in the result set. (See LIMIT) |
OFFSET | Skips a specific number of documents before returning the result set. (See OFFSET) |
Required in each SELECT statement you write in DQL, the FROM element identifies the collection for document retrieval.
For example, a SELECT statement querying documents from the cars collection:
The WHERE clause filters data based on either an expression or a set of conditions that narrow the result set returned to you:
For example, here is a basic SELECT statement querying documents from the cars collection based on a given address:
To demonstrate a more complex query, here is a SELECT statement that queries using multiple expressions and logical operators to further refine the criteria for document retrieval:
With the ORDER BY clause, if you'd like, you can integrate calculations or expressions in your SELECT statement. Then sort the resulting documents to return in either ascending (ASC) or descending (DESC) alphabetical order:
For example, here is a simple SELECT statement that uses the ORDER BY clause to query and sort documents from the cars collection in descending (DESC) alphabetical order based on the field value set for the color property:
In this syntax:
- your_collection_name is the name of the collection from which you want to retrieve the data.
- expression_1, expression_2, ... are the expressions evaluated to sort the result. Expressions are resolved in order.
- [ASC|DESC] is an optional parameter that specifies the sort order. If omitted, the default sort order is ascending (ASC). To sort in descending order, you can specify DESC.
Example
In this example, the result set from the query will be sorted in descending order based on the values in the field:
For instance, here "blue" cars return first and other cars by natural order in the collection:
Comparing objects of different types
Values of differing types are sorted in the following order when the sort order is ascending, order is reversed when descending:
- boolean
- number
- binary
- string
- array
- object
- null
- missing
Unless explicitly defined as DESC in your query, Ditto defaults to sorting in ascending (ASC). So, if you want to sort in ascending order, it is not necessary to express that in your query.
The LIMIT clause is used to restrict the number of documents returned by a query, allowing you to specify a maximum limit on the number of documents to be included in the result set:
In this syntax:
- your_collection_name is the name of the collection from which you want to retrieve the data.
- limit_value is the maximum number of documents you want to include in the result set.
For example, only return the first 10 documents from the your_collection_name collection:
The OFFSET clause is used to specify the number of records to skip before starting to return documents from the query result:
In this syntax:
- your_collection_name is the name of the collection from which you want to retrieve the data.
- number_of_items_to_skip is the number of items before returning the result set.
Using OFFSET with LIMIT is a common way to utilize OFFSET; for example: