Rule SQL Reference

EMQX uses a SQL-based syntax in its rules for data extraction, filtering, enrichment, and transformation. This SQL-like syntax has two types of statements: SELECT and FOREACH.

Statement	Description
SELECT	For situations where the result of the SQL statement is a single message.
FOREACH	For producing zero or more messages from a single input message.

Each rule can have exactly one statement. SQL statements provide a rich set of built-in functions, enabling you to perform simple transformations and create timestamps, among other operations.

SQL statements also supports embedding jq programs within expressions, which allows you to do complex data transformations whenever required. Expressions can be embedded within the SELECT and FOREACH statements. For the fields that can be referenced in the SELECT and FOREACH statements, see Data Sources and Fields.

The SELECT Statement

The SELECT statement selects specific fields from an input message, rename fields, transform data, and filter messages based on conditions.

The basic format of a SELECT statement in the rule engine SQL is as follows:

SELECT <fields_expressions> FROM <topic> [WHERE <conditions>]

You can use the SELECT clause to specify which fields (from both message payload and metadata) to include in the output, and use the WHERE clause to filter messages based on specific conditions

The FROM Clause

The FROM clause is used to specify the data source for your query. You can choose to select data from specific topics or events that match a certain condition.

Select by Topics

For example, if you want to defines a rule applies to all messages published to topics matching the pattern t/# and my/other/topic, you can work with the statement below:

SELECT clientid, payload.clientid as myclientid FROM "t/#", "my/other/topic"

Where,

• the SELECT clause is to specify the fields to be included in the output

  • clientid: is the client ID in the metadata

  • payload.clientid: is the client ID in the message payload. All fields in the message payload are stored under payload.

    • The as syntax renames the payload.clientid field as myclientid.

Select by Events

You can also attach rules to events. For example, if you want to get the IP address and port number when client c1 initiates a connection to EMQX, you can use the statement below:

SELECT peername as ip_port FROM "$events/client_connected" WHERE clientid = 'c1'

TIP

You can find all available events in the EMQX Dashboard (under the Events tab when editing a rule).

The WHERE Clause

The WHERE clause provides an optional way to refine message filtering by specifying additional conditions that the messages must meet, in addition to the topic/event filter specified in the FROM clause.

For instance, the following SQL statement filters messages under topic t/# that are sent by the user name eric:

SELECT * FROM "t/#" WHERE username = 'eric'

TIP

The fields used in the WHERE clause must be one field available in the message metadata or payload, or there will be an error.

Work with Expressions

Expressions can also be used to transform data in the SELECT and WHERE clause. For example, the following SQL statement formats the clientid field value by converting it to uppercase and adding a suffix. The result is named cid in the output message:

SELECT (upper(clientid) + '_UPPERCASE_LETTERS') as cid FROM "t/#"

The following showcase the use of a parenthesized arithmetic expression to transform data:

SELECT (payload.integer_field + 2) * 2 as num FROM "t/#"

You can also use dot notation to access fields in a payload with a complex structure (this assumes that the payload is JSON formatted):

SELECT payload.a.b.c.deep as my_field FROM "t/#"

The statement below shows an example of how the equality operator (=) can be used to test that a field has a certain value in the WHERE-clause. Using * in the SELECT-clause will forward all metadata and the payload to the output message:

SELECT * FROM "t/#" WHERE payload.x.y = 1

You can use the and and or operators to form complex boolean expressions in the WHERE-clause:

SELECT * FROM "t/#" WHERE payload.name = "sensor_1" and payload.temprature > 39

The FOREACH Statement

The FOREACH statement can be seen as a more general form of the SELECT statement. It can produce zero or more output messages for each input message. You can use the FOREACH statement to filter data based on specific conditions and output the results to MQTT topics or data bridges.

The basic format of a FOREACH statement in the rule engine SQL is as follows:

FOREACH <expression_that_evaluates_to_array> [as <name>]
[DO <fields_expressions>]
[INCASE <condition>]
FROM <topic>
[WHERE <condition>]

A FOREACH statement starts with a FOREACH-clause that is used to create an array from the input message. The FROM and WHERE clauses have the same purpose and work the same as the clauses with the corresponding names in the SELECT statement. Besides the FOREACH, FROM and WHERE clauses, the FOREACH statement has two more optional clauses:

Clause	Optional/Required	Description
DO	Optional	Transform each element in the array selected by FOREACH. Correspond to the SELECT clause in the SELECT statement and accepts the same expressions.
INCASE	Optional	Filter out array elements that do not match the specified conditions. Accepts the same expressions as the WHERE clause.

TIP

Except for the FOREACH clause, all other clauses in the FOREACH statement have corresponding clauses in the SELECT statement. This implies that the FOREACH statement can be seen as a generalization of the SELECT statement, as mentioned earlier. The following two statements are equivalent (note that jq('.', payload) wraps the payload inside an array):

FOREACH jq('.', payload) as it
DO it.field_1, it.field_2 
FROM "t/#"

SELECT payload.field_1, payload.field_2
FROM "t/#"

The as-syntax in the FOREACH clause exemplified above is utilized to assign a name to the array items, which allows for easy referencing of the "current" item within the DO clause. If the "as name" part is omitted, the default name "item" will be used.

Below is an example to show how to use the FOREACH statement to output two values. Both values contain only one field called value. The value of the field value is the value of the field field_1 in one of the messages and the value of field_2 in the other message:

FOREACH jq('[.field_1, .field_2]', payload) 
DO item as value
FROM "t/#"

The FOREACH statement requires input data to be in an array format. If the input message already contains an array, you can directly apply the FOREACH statement.

For example, for messages published to topics t/#, if you want to output the timestamp, client ID, sensor name, and index when the sensor idx is 1 or above, you can use the code below:

FOREACH
    payload.sensors as sensor  
DO
    timestamp,
    clientid,
    upper(sensor.name) as name,
    sensor.idx as idx
INCASE
    sensor.idx >= 1
FROM "t/#"

Where,

• The FOREACH-clause specifies the sensors field in the input message's payload as the array to iterate over. It also names the array itmes sensor with the "as nane"-syntax.
• The DO clause specifies the fields to be included in the output:
  • timestamp is the timestamp from the input message's metadata.
  • clientid is the client ID from the input message's metadata.
  • sensor.name will be capitalized with the build-in upper function and renamed as name with the "as name"-syntax. Please note that "sensor" specifically refers to the current item within the array that is selected by the FOREACH clause.
  • sensor.idx will be renamed as idx with the as clause.
• The INCASE clause adds another filter condition, so you only get sensors where the value of the idx-field is 1 and above.
• The FROM clause specifies that you want messages from topics that match the pattern "t/#".

After creating your rules, it's always recommended to test your rules before putting them into production. The Dashboard UI contains a test feature that allows you to test your rules with sample messages. For details on how to test the SQL statements, see Test the Rule. The rule above can be tested by putting in the following JSON formatted text as payload:

{"sensors": [
    {"idx":0, "name":"t0"},
    {"idx":1, "name":"t1"},
    {"idx":2, "name":"t2"}
  ]
}

If the input message does not contain an array, you can use the jq function to wrap the payload in an array, for example, in the code below.

FOREACH jq('.', payload) 
DO item.field_1, item.field_2 
FROM "t/#"

EMQX supports using the jq function for advanced transformations. You can refer to the build-in jq function for more code examples.

Expressions and Operations

EMQX rule syntax allows using expressions to transform data and filter messages, which can be used in various clauses, including SELECT, FOREACH, DO, INCASE, and WHERE. This section offers more information on using these expressions. The following are the operations that can be used to form expressions, and remember that there is a wide range of built-in functions that can also be used in expressions.

Arithmetic Operations

Function	Purpose	Returned value
+	addition, or string concatenation	Sum, or concatenated string
-	Subtraction	Difference
*	multiplication	product
/	division	Quotient
div	Integer division	Integer quotient
mod	modulus	module

Logical Operations

Function	Purpose	Returned Value
>	greater than	true/false
<	less than	true/false
<=	less than or equal	true/false
>=	greater than or equal	true/false
<>	not equal	true/false
!=	not equal	true/false
=	Check if the two operands are completely equal. It can be used to compare values	true/false
=~	Check if a topic can match the topic filter. It can only be used for topic matching	true/false
and	logical and	true/false
or	logical or	true/false

CASE Expressions

The CASE expression can be used to perform conditional operations. A case expression corresponds to an if-then-else statement in other languages. How to use the CASE expression is illustrated by the following example.

SELECT
  CASE WHEN payload.x < 0 THEN 0
       WHEN payload.x > 7 THEN 7
       ELSE payload.x
  END as x
FROM "t/#"

Suppose the message is:

{"x": 8}

Then the output will be:

{"x": 7}

More Examples

Examples of SELECT Statements

• Extract all fields from the messages with the topic "t/a":

  SELECT * FROM "t/a"

• Extract all fields from the messages with the topics "t/a" or "t/b":

  SELECT * FROM "t/a","t/b"

• Extract all fields from the message with a topic that matches 't/#'.

  SELECT * FROM "t/#"

• Extract the qos, username, and clientid fields from the input message with a topic that matches 't/#' (the output message will have a payload with the fields qos, username, and clientid):

  SELECT qos, username, clientid FROM "t/#"

• Extract the username field from any message with a payload field named username with the value 'Steven' (it is not recommended to use the topic filter '#' in the FROM clause as this means that the rule has to be checked for all messages that are sent to EMQX):

  SELECT username FROM "#" WHERE username='Steven'

• Extract the x field from the payload of the input message and rename the field to y in the output message. The new alias y for payload.x can also be used in the WHERE clause. A rule with this SQL statement matches messages with the payload {"x": 1} but not messages with the payload {"x": 2}:

  SELECT payload.x as x FROM "tests/test_topic_1" WHERE y = 1

• This SQL statement matches messages with the payload {"x": {"y": 1}} (and for example {"x": {"y": 1}, "other": "field}):

  SELECT * FROM "#" WHERE payload.x.y = 1

• If an MQTT client with clientid = 'c1' connected, extract its source IP address and port number:

  SELECT peername as ip_port FROM "$events/client_connected" WHERE clientid = 'c1'

• Matches all subscriptions to topics that matches the pattern 't/topic' and have a quality of service (QoS) level of 1. It extracts the clientid to the output message.

  SELECT clientid FROM "$events/session_subscribed" WHERE topic = 'my/topic' and qos = 1

• Similar to the above example but here the topic match operator =~ is used to match the topic filter 't/#':

  SELECT clientid FROM "$events/session_subscribed" WHERE topic =~ 't/#' and qos = 1

• Extract the User Property with Key "foo" (User properties is new in the MQTT 5.0 protocol so this is not relevant for older MQTT versions):

  SELECT pub_props.'User-Property'.foo as foo FROM "t/#"

TIP

• Topics in the FROM clause need to be enclosed in double quotes ("").

• The WHERE clause is followed by the filter condition. If a string is used in the condition, it needs to be enclosed in single quotes ('').

• If there are multiple topics in the FROM clause, they need to be separated by commas ",". For example, SELECT * FROM "topic1", "topic2" WHERE topic1.id = topic2.id.

  SELECT * FROM "t/1", "t/2".

• You can use the period symbol (.) to access inner fields of the payload. For example, if the payload is a nested JSON structure, you can use payload.outer_field.inner_field to access the inner_field of the outer_field.

Examples of FOREACH Statements

Suppose there are messages with client ID c_steve coming to the topic t/1. The message body is in JSON format, and the sensors field is an array containing multiple objects as is shown in the following example:

{
    "date": "2020-04-24",
    "sensors": [
        {"name": "a", "idx":0},
        {"name": "b", "idx":1},
        {"name": "c", "idx":2}
    ]
}

Example 1

In this example it is required that each object in sensors array is re-published to the topic sensors/${idx} (where index is taken from the object) with the content of ${name} (where name is taken from the object). That means that for for the example input given above the rule engine will issue the following three messages:

1. Topic: sensors/0 Content: a
2. Topic: sensors/1 Content: b
3. Topic: sensors/2 Content: c

For the rule in this example, we need to configure the following action:

• Action type: message republish
• Target topic: sensors/${idx}
• Target QoS: 2
• Message content template: ${name}

And the following SQL statement:

FOREACH
    payload.sensors
FROM "t/#"

In the SQL statement above, the FOREACH clause specifies the array sensors that needs to be traversed. The FOREACH statement will perform a "message republish" action for each object in the result array, so the republish action will be performed three times.

Example 2

In this example, it is required that each object in the sensors array with a value for the id field which is greater than or equal to 1 is re-published to the topic sensors/${idx} with the content clientid=${clientid},name=${name},date=${date}. This means that the rule will issue two messages when given the example input message specified above (since the array element with the id field set to zero will be filtered out).

1. Topic: sensors/1 Content: clientid=c_steve,name=b,date=2023-04-24
2. Topic: sensors/2 Content: clientid=c_steve,name=c,date=2023-04-24

For the rule in this example, we need to configure the following action:

• Action type: message republish
• Target topic: sensors/${idx}
• Target QoS: 2
• Message content template: clientid=${clientid},name=${name},date=${date}

And the following SQL statement:

FOREACH
    payload.sensors
DO
    clientid,
    item.name as name,
    item.idx as idx
INCASE
    item.idx >= 1
FROM "t/#"

In the above SQL statement, the FOREACH clause specifies that the array sensors needs to be traversed. The DO clause selects the fields required for each operation. The clientid field is selected from the message meta data and name and idx are selected from the current sensor object. The name item represents the current object in the sensors array. The INCASE clause specifies a filter condition for the array objects (objects that do not match the filter will be ignored).

In DO and INCASE clauses, you can use item to access the current object, or you can customize a variable name by using the as syntax in FOREACH. So the SQL statement in this example can also be written as follows:

FOREACH
    payload.sensors as s
DO
    clientid,
    s.name as name,
    s.idx as idx
INCASE
    s.idx >= 1
FROM "t/#"

Example 3

This extends Example 2 by also removing the c_ prefix of c_steve in the clientid field.

The rule engine comes with a number of built in functions can be called in the FOREACH, DO and INCASE clauses. If you want to change c_steve into steve, you can change the SQL in Example 2 into:

FOREACH
    payload.sensors as s
DO
    nth(2, tokens(clientid,'_')) as clientid,
    s.name as name,
    s.idx as idx
INCASE
    s.idx >= 1
FROM "t/#"

Multiple expressions can be placed in the FOREACH clause as long as the last expression specifies the array to be traversed. For example, if the input messages payload was formatted like this instead:

{
    "date": "2020-04-24",
    "data": {
        "sensors": [
            {"name": "a", "idx":0},
            {"name": "b", "idx":1},
            {"name": "c", "idx":2}
        ]
    }
}

Then the FOREACH clause can give the payload data another name before selecting the array:

FOREACH
    payload.data as d
    d.sensors as s
...

This is equivalent to:

FOREACH
    payload.data.sensors as s
...

This feature can be useful when you are working with payloads that are structured in complex ways.