SQL Overview

    The SQL grammar of HStreamDB (opens new window) is based on a subset of SQL-92 with some extensions to support stream operations.

    SQL inputs are made up of a series of statements. Each statement is made up of a series of tokens and ends in a semicolon ().

    A token can be a keyword argument, an identifier, a literal, an operator, or a special character. The details of the rules can be found in the . Normally, tokens are separated by whitespace.

    The following examples are syntactically valid SQL statements:

    Keywords

    Some tokens such as SELECT, INSERT and WHERE are reserved keywords, which have specific meanings in SQL syntax. Keywords are case insensitive, which means that SELECT and select are equivalent. A keyword can not be used as an identifier.

    For a complete list of keywords, see the .

    Identifiers are tokens that represent user-defined objects such as streams, fields, and other ones. For example, my_stream can be used as a stream name, and temperature can represent a field in the stream.

    By now, identifiers are case sensitive, which means that my_stream and MY_STREAM are different identifiers.

    Literals (Constants)

    Literals are objects with known values before being executed. There are six types of constants: integers, floats, strings, dates, time, and intervals so far.

    Integers are in the form of digits, where digits are one or more single-digit integers (0 through 9). Negatives such as -1 are also supported. Note that scientific notation is not supported yet.

    Floats

    Floats are in the form of digits . digits. Negative floats such as -11.514 are supported. Note that

    • Forms such as 1. and .99 are not supported yet.

    Strings are arbitrary character series surrounded by double quotes ("), such as "JSON".

    Dates

    Dates represent a date exact to a day in the form of , where <year>, <month> and <day> are all integer constants. Note that the leading DATE should not be omitted.

    Example: DATE 2021-01-02

    Example: TIME 11:45:14

    Intervals

    Intervals represent a time section in the form of INTERVAL <num> <time_unit>, where <num> is an integer constant and <time_unit> is one of YEAR, MONTH, WEEK, DAY, and SECOND. Note that the leading INTERVAL should not be omitted.

    Example: INTERVAL 5 SECOND

    Functions are special keywords that mean some computation, such as SUM and MIN. And operators are infix functions composed of special characters, such as >= and <>.

    For a complete list of functions and operators, see the appendix.

    Special characters

    There are some special characters in the SQL syntax with particular meanings:

    • Parentheses (()) are used outside an expression for controlling the order of evaluation or specifying a function application.
    • Brackets ([]) are used with maps and arrays for accessing their substructures, such as some_map[temp] and some_array[1]. Note that it is not supported yet.
    • Commas (,) are used for delineating a list of objects.
    • The semicolons (;) represent the end of a SQL statement.
    • The asterisk (*) represents “all fields”, such as SELECT * FROM my_stream;.

    A single-line comment begins with //: