GQL Reference

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GQL maps roughly to SQL: you can think of GQL kind as a SQL table, a GQL entity as a SQL row, and a GQL property as a SQL column. However, a SQL row-column lookup is limited to a single value, whereas in GQL a property can be a multiple value property.

Grammar

The GQL grammar is summarized as follows:

<query> :=
SELECT ( * | [ DISTINCT ] <aggregated-property>+, )
[ FROM <kind> ]
[ WHERE <compound-condition> ]
[ GROUP BY <property-name>+,  ]
[ ORDER BY ( <property-name> [ ASC | DESC ] )+,  ]
[ LIMIT ( <result-position> |
          FIRST “(” <result-position> ,
                    <result-position> “)” ) ]
[ OFFSET <result-position> [ “+” <result-position> ] ]

<aggregated-property> :=
  <property-name>
| aggregator “(” <property-name> “)”

<aggregator> := FIRST

<compound-condition> := <condition>+AND

<condition> :=
  <property-name> IS NULL
| <property-name> <comparator> <value>
| <value> <comparator> <property-name>

<comparator> :=
  =
| <
| <=
| >
| >=
| IN
| CONTAINS
| HAS ANCESTOR
| HAS DESCENDANT

<result-position> := <binding-site> | <integer-literal>

<value> :=
  <binding-site>
| <synthetic-literal>
| <string-literal>
| <integer-literal>
| <double-literal>
| <boolean-literal>
| <null-literal>

<synthetic-literal> :=
  KEY “(”
    [ “DATASET” “(“ <string-literal> “)” “,” ]
    [ “NAMESPACE” “(“ <string-literal> “)” “,” ]
    <key-path-element>+, “)”
| BLOB “(” <string-literal> “)”
| BLOBKEY “(” <string-literal> “)”
| DATETIME “(” <string-literal> “)”

<key-path-element> :=
  <kind> "," ( <integer-literal> | <string-literal> )

<kind> := <name>

<property-name> := <name>

In the above grammar list, note that:

• The symbol +, after an expression indicates that it can be repeated, with repeated expressions separated by a comma.
• The use of +AND above means an AND-separated set of conditions.

The following are some examples that return entire entities:

    SELECT * FROM myKind WHERE myProp >= 100 AND myProp < 200
    SELECT * FROM myKind LIMIT 50 OFFSET @startCursor 
    SELECT * FROM myKind ORDER BY myProp DESC

Every GQL query string always begins with SELECT <something>, where <something> is one of the following:

• *
• <property-list>, a comma delimited list of properties to be returned from the query.
• __key__, which returns keys only.

Similar to SQL, GQL keywords are case insensitive. Kind, property, and binding site names are case sensitive.

A GQL query returns zero or more entity results of the requested kind, with each result consisting of an entire entity or some subset of the properties of the entity, or even just the entity key. For example,

• SELECT * FROM myKind
• SELECT __key__ FROM myKind
• SELECT title, year FROM Song WHERE composer = 'Lennon, John'

A result list produced by SELECT * or SELECT __key__ never contains duplicates. A result list produced by SELECT <property-list> may contain multiple results from one entity, typically when any of those properties are multiple value properties.

Tip: SELECT __key__ or SELECT <property-list> queries are faster than SELECT * queries, and are cheaper, because they are charged at the less expensive small operations rates, rather than at the read rates. See also projection queries.

Clauses

The following optional SELECT clauses are recognized:

How to form entity and property names

Kind, property, and binding site names are formed as follows:

• With any sequence of letters, digits, underscores, dollar signs, or unicode characters in the range from U+0080 to U+FFFF (inclusive), so long as the name does not begin with a digit. For example, foo, bar17, x_y, big$bux, __qux__.

• You can also use a non-empty backquoted string: `fig-bash` or `x.y`. A backquote character can be represented in a backquoted name by doubling it, for example, `silly``putty`. A backquoted name can contain escaped characters.

• An unquoted name can match a predefined name, but must not match a keyword. A backquoted name can contain any character except a newline. (It can contain a newline via \n, but not as a raw newline.)

• Names are case-sensitive.

How to form literals

You can use the following literals in a comparison:

Synthetic literals

A synthetic literal is a value that is constructed by a function. The following table lists the supported synthetic literals:

How to escape characters

You can escape certain characters in string literals and backquoted names. The escaped characters are case sensitive: for example \r is valid while \R is not.

The following is a list of all the characters that can be escaped in GQL:

Operators and comparisons

Comparators are either equivalence comparators: =, IN, CONTAINS, = NULL, HAS ANCESTOR, and HAS DESCENDANT, or inequality comparators: <, <=, >, and >=.

Notice that the operator = is another name for the IN and CONTAINS operators. For example, <value> = <property-name> is the same as <value> IN <property-name>, and <property-name> = <value> is the same as <property-name> CONTAINS <value>. Also <property-name> IS NULL is the same as <property-name> = NULL.

A condition can also test whether one entity has another entity as an ancestor, using the HAS ANCESTOR or HAS DESCENDANT operators. These operators test ancestor relationships between keys. They can operate on __key__, but they can also operate on a key-valued property. For HAS ANCESTOR, the right operand cannot be a property. For HAS DESCENDANT, the left operand cannot be a property. For more information on ancestor relationships, see ancestor paths.

Only one property may be compared with inequality operators. When a query with an ORDER BY clause applies an inequality operator to a property, that property must be the first property in the ORDER BY clause.

A typical property name consists of alphanumeric characters optionally mixed with underscore (_) and dollar sign ($). In other words, they match the regular expression [a-zA-Z0-9_$]+(.[0-9_$]+)*. Property names containing other printable characters must be quoted with backquotes, for example: `first-name`.

Restrictions

Comparisons must be between a property name and a literal, but these can be on either side of the operator. For example, A < 7 or 7 > A. Note, however, that there is no inverse operator for IS NULL, so while you can have <property-name> IS NULL, you cannot have NULL IS <property-name>.

There is no way to determine whether an entity lacks a value for a property (that is, whether the property has no value). If you use a condition of the form property = NULL, what will occur is a check whether a null value is explicitly stored for that property. Datastore queries that refer to a property will never return entities that don't have a value for that property.

Examples

To find all of the entities of kind Person whose ages are between 18 and 35, use this query string:

SELECT * FROM Person WHERE age >= 18 AND age <= 35

To find the three entities of kind Person whose ages are the greatest, use this query string:

SELECT * FROM Person ORDER BY age DESC LIMIT 3

To return only the name property for each Person, use this query string:

SELECT name FROM Person

To return only the name property for each Person, ordered by age, use this query string:

SELECT name FROM Person ORDER BY age

To find the keys of the entities of kind Person that have an age of NULL, use this query string:

SELECT __key__ FROM Person WHERE age = NULL

To find all the entities, regardless of kind, that are in Amy's entity group (i.e. Amy and Fred), use this query:

SELECT * WHERE __key__ HAS ANCESTOR KEY(Person, 'Amy')

Argument binding

GQL supports argument binding. For information about these, see Using GQL.

Unsupported features and behavior differences from MySQL/Python GQL

If you are familiar with MySQL or the Python GQL provided by Google App Engine, you might want to take a look at the following list, which highlights the main differences between those products and the Datastore GQL behavior.

Keywords and predefined names

Keywords and predefined names are case-insensitive.

The following keywords are recognized, although not all of these are currently used in GQL. The unused keywords are marked with an asterisk.

The following predefined names are recognized:

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