FROM Clause

Capella Operational

reference

The FROM clause specifies the documents to be used as the input for a query.

Purpose

The FROM clause is used within a SELECT query or subquery. It specifies the documents to be used as the input for a query.

Prerequisites

For you to select data from keyspace or expression, you must have the query_select privilege on that keyspace. For more details about user roles, see Authorization.

Syntax

from-clause ::= 'FROM' from-terms

FROM Terms

from-terms ::= ( from-keyspace | from-subquery | from-generic )
               ( join-clause | nest-clause | unnest-clause )* comma-separated-join*

The first FROM term may be any of the following:

A keyspace reference
A subquery (such as derived tables)
A generic expression (nested paths, CURL(), or other expressions)

This may be followed by further FROM terms, each of which may be one of the following:

A JOIN clause and conditions
A NEST clause and conditions
An UNNEST clause and conditions

You may additionally include one or more comma-separated joins.

JOIN clauses, NEST clauses, UNNEST clauses, and comma-separated joins each have a left-hand side and a right-hand side. The left-hand side is defined by the preceding FROM term; the right-hand side is defined by the FROM term itself.

When you chain multiple FROM terms together, the right-hand side of one FROM term acts as the left-hand side of the following FROM term.

Limitations

When the FROM term is an expression, USE KEYS or USE INDEX clauses are not allowed.
When using a lookup JOIN clause, an index JOIN clause, a NEST clause, or an UNNEST clause, the left-hand side of the join may be a keyspace identifier, an expression, or a subquery; but the right-hand side may only be a keyspace identifier.
When using an ANSI JOIN clause, the right-hand side of the join may also be a keyspace identifier, an expression, or a subquery, similar to the left-hand side.
You can chain comma-separated joins with ANSI JOIN clauses, ANSI NEST clauses, and UNNEST clauses. However, you cannot chain comma-separated joins with lookup JOIN and NEST clauses, or index JOIN and NEST clauses.
The right-hand side of a comma-separated join can only be a keyspace identifier, a subquery, or a generic expression. This means that comma-separated joins must come after any JOIN, NEST, or UNNEST clauses.

FROM Keyspace

The FROM keyspace specifies a keyspace to query from: either a specific keyspace or a constant expression.

Syntax

from-keyspace ::= keyspace-ref ( 'AS'? alias )? use-clause?

keyspace-ref	Keyspace Reference
alias	AS Alias
use-clause	USE Clause

Keyspace Reference

keyspace-ref ::= keyspace-path | keyspace-partial

keyspace-path	Keyspace Path
keyspace-partial	Keyspace Partial

Keyspace reference of the data source. The identifiers that make up the keyspace reference are not available as variables in scope of a subquery.

If there is a hyphen (-) inside any part of the keyspace reference, you must wrap that part of the keyspace reference in backticks (` `). Refer to the examples below.

Keyspace Path

keyspace-path ::= ( namespace ':' )? bucket ( '.' scope '.' collection )?

If the keyspace is a named collection, or the default collection in the default scope within a bucket, the keyspace reference may be a keyspace path. In this case, the query context should not be set.

namespace: (Optional) An identifier that refers to the namespace of the keyspace. Currently, only the default namespace is available. If the namespace name is omitted, the default namespace in the current session is used.
bucket: (Required) An identifier that refers to the bucket name of the keyspace.
scope: (Optional) An identifier that refers to the scope name of the keyspace. If omitted, the bucket’s default scope is used.
collection: (Optional) An identifier that refers to the collection name of the keyspace. If omitted, the default collection in the bucket’s default scope is used.

For example, default:`travel-sample` indicates the default collection in the default scope in the travel-sample bucket in the default namespace.

Similarly, default:`travel-sample`.inventory.airline indicates the airline collection in the inventory scope in the travel-sample bucket in the default namespace.

Keyspace Partial

keyspace-partial ::= collection

Alternatively, if the keyspace is a named collection, the keyspace reference may be just the collection name with no path. In this case, you must set the query context to indicate the required namespace, bucket, and scope.

collection: (Required) An identifier that refers to the collection name of the keyspace.

For example, airline indicates the airline collection, assuming the query context is set.

AS Alias

Assigns another name to the FROM keyspace. For details, see AS Clause.

Assigning an alias is optional for the FROM keyspace. If you assign an alias to the FROM keyspace, the AS keyword may be omitted.

USE Clause

Enables you to specify that the query should use particular keys, or a particular index. For details, see USE clause.

Examples

To try the examples in this section, set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

The simplest type of FROM keyspace clause specifies a single keyspace.

Example 1. Use a single keyspace

Select four unique landmarks from the landmark keyspace.

SELECT DISTINCT name
FROM landmark
LIMIT 4;

Results

[
  {
    "name": "Royal Engineers Museum"
  },
  {
    "name": "Hollywood Bowl"
  },
  {
    "name": "Thai Won Mien"
  },
  {
    "name": "Spice Court"
  }
]

FROM Subquery

Specifies a SQL++ SELECT expression of input objects.

Syntax

from-subquery ::= subquery-expr 'AS'? alias

subquery-expr	Subquery Expression
alias	AS Alias

Subquery Expression

subquery-expr ::= '(' select ')'

Use parentheses to specify a subquery.

For more details and examples, see SELECT Clause and Subqueries.

AS Alias

Assigns another name to the subquery. For details, see AS Clause.

Assigning an alias is required for subqueries in the FROM term. However, when you assign an alias to the subquery, the AS keyword may be omitted.

Examples

To try the examples in this section, set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

Example 2. A SELECT clause inside a FROM clause.

List all Gillingham landmark names from a subset of all landmark eating places.

SELECT name, city
FROM (SELECT id, name, address, city
      FROM landmark
      WHERE activity = "eat") AS l
WHERE city = "Gillingham";

Results

[
  {
    "city": "Gillingham",
    "name": "Hollywood Bowl"
  },
  {
    "city": "Gillingham",
    "name": "Thai Won Mien"
  },
  {
    "city": "Gillingham",
    "name": "Spice Court"
  },
  {
    "city": "Gillingham",
    "name": "Beijing Inn"
  },
  {
    "city": "Gillingham",
    "name": "Ossie's Fish and Chips"
  }
]

Example 3. Subquery Example

For each country, find the number of airports at different altitudes and their corresponding cities.

In this case, the inner query finds the first level of grouping of different altitudes by country and corresponding number of cities. Then the outer query builds on the inner query results to count the number of different altitude groups for each country and the total number of cities.

SELECT t1.country, num_alts, total_cities
FROM (SELECT country, geo.alt AS alt,
             count(city) AS num_cities
      FROM airport
      GROUP BY country, geo.alt) t1
GROUP BY t1.country
LETTING num_alts = count(t1.alt), total_cities = sum(t1.num_cities);

Results

[
  {
    "country": "United States",
    "num_alts": 946,
    "total_cities": 1560
  },
  {
    "country": "United Kingdom",
    "num_alts": 128,
    "total_cities": 187
  },
  {
    "country": "France",
    "num_alts": 196,
    "total_cities": 221
  }
]

This is equivalent to blending the results of the following two queries by country, but the subquery in the from-term above simplified it.

SELECT country,count(city) AS num_cities
FROM airport
GROUP BY country;

SELECT country, count(distinct geo.alt) AS num_alts
FROM airport
GROUP BY country;

FROM Generic Expression

Generic expressions in the FROM term may include SQL++ functions, operators, path expressions, language constructs on constant expressions, variables, and subqueries. This adds huge flexibility by enabling just about any FROM clause imaginable.

Syntax

from-generic ::= expr ( 'AS' alias )?

expr	A SQL++ expression generating JSON documents or objects.
alias	AS Alias

AS Alias

Assigns another name to the generic expression. For details, see AS Clause.

Assigning an alias is optional for generic expressions in the FROM term. However, when you assign an alias to the expression, the AS keyword is required.

Examples

To try the examples in this section, set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

Example 4. Independent Constant Expression

The expression may include JSON scalar values, static JSON literals, objects, or SQL++ functions.

SELECT * FROM [1, 2, "name", { "type" : "airport", "id" : "SFO"}] AS ks1;

SELECT CURL("https://maps.googleapis.com/maps/api/geocode/json",
           {"data":"address=Half+Moon+Bay" , "request":"GET"} );

Note that functions such as CURL() can independently produce input data objects for the query. Similarly, other SQL++ functions can also be used in the expressions.

Example 5. Variable SQL++ Expression

The expression may refer to any variables in scope for the query.

SELECT count(*)
FROM airport t
LET x = t.geo
WHERE (SELECT RAW y.alt FROM x y)[0] > 6000;

The FROM x clause is an expression that refers to the outer query. This is applicable to only subqueries because the outermost level query cannot use any variables in its own FROM clause. This makes the subquery correlated with outer queries, as explained in the Subqueries section.

AS Clause

To use a shorter or clearer name anywhere in the query, like SQL, SQL++ allows you to assign an alias to any FROM term in the FROM clause.

Syntax

The AS keyword is required when assigning an alias to a generic expression.

The AS keyword is optional when assigning an alias to the FROM keyspace, a subquery, the JOIN clause, the NEST clause, or the UNNEST clause.

Arguments

alias: String to assign an alias.

Since the original name may lead to referencing wrong data and wrong results, you must use the alias name throughout the query instead of the original keyspace name.

In the FROM clause, the renaming appears only in the projection and not the fields themselves.

When no alias is used, the keyspace or last field name of an expression is given as the implicit alias.

When an alias conflicts with a keyspace or field name in the same scope, the identifier always refers to the alias. This allows for consistent behavior in scenarios where an identifier only conflicts in some documents. For more information on aliases, see Identifiers.

Examples

To try the examples in this section, set the query context to the inventory scope in the travel sample dataset. For more information, see Query Context.

The following FROM clauses are equivalent, with and without the AS keyword.

FROM airport AS t

FROM airport t

FROM hotel AS h
INNER JOIN landmark AS l
ON (h.city = l.city)

FROM hotel h
INNER JOIN landmark l
ON (h.city = l.city)

FROM Clause

Purpose

Prerequisites

Syntax

FROM Terms

Limitations

FROM Keyspace

Syntax

Keyspace Reference

Keyspace Path

Keyspace Partial

AS Alias

USE Clause

Examples

FROM Subquery

Syntax

Subquery Expression

AS Alias

Examples

FROM Generic Expression

Syntax

AS Alias

Examples

AS Clause

Syntax

Arguments

Examples

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