# Query language elements

eKuiper provides a variety of elements for building queries. They are summarized below.

Element	Summary
SELECT	SELECT is used to retrieve rows from input streams and enables the selection of one or many columns from one or many input streams in eKuiper.
FROM	FROM specifies the input stream. The FROM clause is always required for any SELECT statement.
JOIN	JOIN is used to combine records from two or more input streams. JOIN includes LEFT, RIGHT, FULL & CROSS. Join can apply to multiple streams join or stream/table join. To join multiple streams, it must run within a window.
WHERE	WHERE specifies the search condition for the rows returned by the query.
GROUP BY	GROUP BY groups a selected set of rows into a set of summary rows grouped by the values of one or more columns or expressions. It must run within a window.
ORDER BY	Order the rows by values of one or more columns.
HAVING	HAVING specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT expression.

# SELECT

Retrieves rows from input streams and enables the selection of one or many columns from one or many input streams in eKuiper.

# Syntax

SELECT 
	*
	| [source_stream.]column_name [AS column_alias]
	| expression

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# Arguments

Specifies that all columns from all input streams in the FROM clause should be returned. The columns are returned by input source, as specified in the FROM clause, and in the order in which they exist in the incoming stream or specified by ORDER BY clause.

Select all of fields from source stream.

source_stream

The source stream name or alias name.

column_name

Is the name of a column to return. If the column to specified is a embedded nest record type, then use the JSON expressions to refer the embedded columns.

column_alias

Is an alternative name to replace the column name in the query result set. Aliases are used also to specify names for the results of expressions. column_alias cannot be used in a WHERE, GROUP BY, or HAVING clause.

expression

Expression is a constant, function, any combination of column names, constants, and functions connected by an operator or operators.

# FROM

Specifies the input stream. The FROM clause is always required for any SELECT statement.

# Syntax

FROM source_stream | source_stream AS source_stream_alias

# Arguments

source_stream | source_stream_alias

The input stream name or alias name.

# JOIN

JOIN is used to combine records from two or more input streams. JOIN includes LEFT, RIGHT, FULL & CROSS.

# Syntax

LEFT | RIGHT | FULL | CROSS 
JOIN 
source_stream | source_stream AS source_stream_alias
ON <source_stream|source_stream_alias>.column_name =<source_stream|source_stream_alias>.column_name

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# Arguments

LEFT

The LEFT JOIN keyword returns all records from the left stream (stream1), and the matched records from the right stream (stream2). The result is NULL from the right side, if there is no match.

SELECT column_name(s)
FROM stream1
LEFT JOIN stream2
ON stream1.column_name = stream2.column_name;

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RIGHT

The RIGHT JOIN keyword returns all records from the right stream (stream2), and the matched records from the left stream (stream1). The result is NULL from the left side, when there is no match.

SELECT column_name(s)
FROM stream1
RIGHT JOIN stream2
ON stream1.column_name = stream2.column_name;

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FULL

The FULL JOIN keyword return all records when there is a match in left (stream1) or right (stream2) table records.

Note: FULL JOIN can potentially return large result-sets!

SELECT column_name(s)
FROM stream1
FULL JOIN stream2
ON stream1.column_name = stream2.column_name
WHERE condition;

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CROSS

The CROSS JOIN is used to combine each row of the first stream (stream1) with each row of the second stream (stream2). It is also known as the Cartesian join since it returns the Cartesian product of the sets of rows from the joined tables. Let's say if there are m rows in stream1, and n rows in stream2, then the result of CROSS JOIN returns m*n rows.

Note: CROSS JOIN can potentially return very large result-sets!

SELECT column_name(s)
FROM stream1
CROSS OUTER JOIN stream2
ON stream1.column_name = stream2.column_name
WHERE condition;

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source_stream | source_stream_alias

The input stream name or alias name to be joined.

column_name

Is the name of a column to return. If the column to specified is a embedded nest record type, then use the JSON expressions to refer the embedded columns.

# WHERE

WHERE specifies the search condition for the rows returned by the query. The WHERE clause is used to extract only those records that fulfill a specified condition.

# Syntax

WHERE <search_condition>
<search_condition> ::=   
    { <predicate> | ( <search_condition> ) }   
    [ { AND | OR } { <predicate> | ( <search_condition> ) } ]   
[ ,...n ]   
<predicate> ::=   
    { expression { = | < > | ! = | > | > = | < | < = } expression

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# Arguments

Expression is a constant, function, any combination of column names, constants, and functions connected by an operator or operators.

< search_condition >

Specifies the conditions for the rows returned in the result set for a SELECT statement or query expression. There is no limit to the number of predicates that can be included in a search condition.

AND

Combines two conditions and evaluates to TRUE when both of the conditions are TRUE.

Combines two conditions and evaluates to TRUE when either condition is TRUE.

< predicate >

Is an expression that returns TRUE or FALSE.

expression

Is a column name, a constant, a function, a variable, a scalar subquery, or any combination of column names, constants, and functions connected by an operator or operators, or a subquery. The expression can also contain the CASE expression.

Is the operator used to test the equality between two expressions.

Is the operator used to test the condition of two expressions not being equal to each other.

Is the operator used to test the condition of one expression being greater than the other.

Is the operator used to test the condition of one expression being greater than or equal to the other expression.

Is the operator used to test the condition of one expression being less than the other.

Is the operator used to test the condition of one expression being less than or equal to the other expression.

SELECT column1, column2, ...
FROM table_name
WHERE condition;

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# GROUP BY

GROUP BY groups a selected set of rows into a set of summary rows grouped by the values of one or more columns or expressions.

# Syntax

GROUP BY <group by spec>  
  
<group by spec> ::=  
    <group by item> [ ,...n ]  
    | <window_type>  
  
<group by item> ::=  
    <column_expression>

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# Arguments

<window_type>

Specifies any eKuiper supported Windowing, see windows for more info.

< column_expression >

Is the expression or the name of the column on which the grouping operation is performed. The column expression cannot contain a column alias that is defined in the SELECT list.

SELECT column_name(s)
FROM stream1
GROUP BY column_name

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# HAVING

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions. Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT expression. HAVING is typically used in a GROUP BY clause.

# Syntax

[ HAVING <search condition> ]

# Arguments

< search_condition >

Specifies the search condition for the group or the aggregate to meet.

SELECT temp AS t, name FROM topic/sensor1 WHERE name = "dname" GROUP BY name HAVING count(name) > 3

# ORDER BY

Order the rows by values of one or more columns.

# Syntax

ORDER BY column1, column2, ... ASC|DESC

The ORDER BY statement in sql is used to sort the fetched data in either ascending or descending according to one or more columns.

By default ORDER BY sorts the data in ascending order.
The keyword DESC is used to sort the data in descending order and the keyword ASC to sort in ascending order.

# Arguments

ASC

To sort the data in ascending order.

DESC

To sort the data in descending order.

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

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# Case Expression

The case expression evaluates a list of conditions and returns one of multiple possible result expressions. It let you use IF ... THEN ... ELSE logic in SQL statements without having to invoke procedures.

There are two types of case expression: simple case expression and searched case expression.

# Simple Case Expression

The simple case expression compares an expression to a set of simple expressions to determine the result.

# Syntax

CASE value   
     WHEN conditionValue THEN result_expression [ ...n ]   
     [ ELSE else_result_expression ]   
END

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Example:

SELECT CASE color 
    WHEN "red" THEN 1 
    WHEN "yellow" THEN 2 
    ELSE 3 END as colorInteger, 
humidity FROM tbl

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# Searched Case Expression

The searched case expression evaluates a set of bool expressions to determine the result.

# Syntax

CASE    
     WHEN condition THEN result_expression [ ...n ]   
     [ ELSE else_result_expression ]   
END

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Example:

SELECT CASE 
    WHEN size < 150 THEN "S" 
    WHEN size < 170 THEN "M"
    WHEN size < 175 THEN "L"
    ELSE "XL" END as sizeLabel
FROM tbl

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# Use reserved keywords or special characters

If you'd like to use reserved keywords or special characters in rule SQL or streams management, please refer to eKuiper lexical elements.

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