# Query language elements eKuiper provides a variety of elements for building queries. They are summarized below. | Element | Summary | |-----------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [SELECT](#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) | FROM specifies the input stream. The FROM clause is always required for any SELECT statement. | | [JOIN](#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](./windows.md). | | [WHERE](#where) | WHERE specifies the search condition for the rows returned by the query. | | [GROUP BY](#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](./windows.md). | | [ORDER BY](#order-by) | Order the rows by values of one or more columns. | | [HAVING](#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 ```sql SELECT * | [source_stream.]column_name [AS column_alias] | expression ``` ### 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](json_expr.md) 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 ```sql 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 ```sql LEFT | RIGHT | FULL | CROSS JOIN source_stream | source_stream AS source_stream_alias ON .column_name =.column_name ``` ### 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. ```sql SELECT column_name(s) FROM stream1 LEFT JOIN stream2 ON stream1.column_name = stream2.column_name; ``` **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. ```sql SELECT column_name(s) FROM stream1 RIGHT JOIN stream2 ON stream1.column_name = stream2.column_name; ``` **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! ```sql SELECT column_name(s) FROM stream1 FULL JOIN stream2 ON stream1.column_name = stream2.column_name WHERE condition; ``` **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! ```sql SELECT column_name(s) FROM stream1 CROSS OUTER JOIN stream2 ON stream1.column_name = stream2.column_name WHERE condition; ``` **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](json_expr.md) 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 ```text WHERE ::= { | ( ) } [ { AND | OR } { | ( ) } ] [ ,...n ] ::= { expression { = | < > | ! = | > | > = | < | < = } expression ``` ### 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. **OR** 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 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. **[NOT] BETWEEN** Is the operator used to test the condition of one expression in (not) within the range specified. ```sql expression [NOT] BETWEEN expression1 AND expression2 ``` **[NOT] LIKE** Is the operator used to check if the STRING in the first operand matches a pattern specified by the second operand. Patterns can contain these characters: - "%" matches any number of characters. - "_" matches a single character. ```sql expression [NOT] LIKE expression1 ``` Example: ```sql a LIKE "string%" ``` **[NOT] IN** Is the operator used to test the condition of one expression (not) being part of to the other expression. support these two formats ```sql expression [NOT] IN (expression2,...n) ``` *Note*： support multiple expressions at the same time, but each expression must return single value ```sql expression [NOT] IN expression2 ``` *Note*：user must make sure the result of expression2 is in array format ```sql SELECT column1, column2, ... FROM table_name WHERE condition; ``` ## 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 ```sql GROUP BY ::= [ ,...n ] | ::= ``` ## Arguments **** Specifies any eKuiper supported Windowing, see [windows](windows.md) 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. ```sql SELECT column_name(s) FROM stream1 GROUP BY column_name ``` ### 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 ```sql [ HAVING ] ``` #### Arguments **< search_condition >** Specifies the search condition for the group or the aggregate to meet. ```sql 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 ```sql 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. ```sql SELECT column1, column2, ... FROM table_name ORDER BY column1, column2, ... ASC|DESC; ``` ## 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 ```sql CASE value WHEN conditionValue THEN result_expression [ ...n ] [ ELSE else_result_expression ] END ``` **Example**: ```sql SELECT CASE color WHEN "red" THEN 1 WHEN "yellow" THEN 2 ELSE 3 END as colorInteger, humidity FROM tbl ``` ### Searched Case Expression The searched case expression evaluates a set of bool expressions to determine the result. #### Syntax ```sql CASE WHEN condition THEN result_expression [ ...n ] [ ELSE else_result_expression ] END ``` **Example**: ```sql SELECT CASE WHEN size < 150 THEN "S" WHEN size < 170 THEN "M" WHEN size < 175 THEN "L" ELSE "XL" END as sizeLabel FROM tbl ``` ## 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](lexical_elements.md).