# SELECT (Stream)

Get records from a materialized view or a stream. Note that SELECT from streams can only be used as a part of CREATE STREAM or CREATE VIEW. Unless when there are cases you would want to run an interactive query from the command shell, you could add EMIT CHANGES at the end of the following examples.

# Synopsis

SELECT <* | identifier.* | expression [ AS field_alias ] [, ...]>
  FROM stream_ref
  [ WHERE expression ]
  [ GROUP BY field_name [, ...] ]
  [ HAVING expression ];

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# Notes

# About `expression`

expression can be any expression described here, such as temperature, weather.humidity, 114514, 1 + 2, SUM(productions), `COUNT(*)` and even subquery SELECT * FROM stream_test WHERE a > 1. In WHERE and HAVING clauses, expression should have a value of boolean type.

# About `stream_ref`

stream_ref specifies a source stream or materialized view:

  stream_ref ::= <identifier>
              | <stream_ref> AS <identifier>
              | <stream_ref> <join_type> <stream_ref> <join_condition> WITHIN Interval
              | <time_window_function> ( <identifier> )

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It seems quite complex! Do not worry. In a word, a stream_ref is something you can retrieve data from. A stream_ref can be an identifier, a join of two stream_refs, a stream_ref with a time window or a stream_ref with an alias. We will describe them in detail.

# JOIN

Fortunately, the JOIN in our SQL query is the same as the SQL standard, which is used by most of your familiar databases such as MySQL and PostgreSQL. It can be one of:

CROSS JOIN, which produces the Cartesian product of two streams and/or materialized view(s). It is equivalent to INNER JOIN ON TRUE.
[INNER] JOIN, which produces all data in the qualified Cartesian product by the join condition. Note a join condition must be specified.
LEFT [OUTER] JOIN, which produces all data in the qualified Cartesian product by the join condition plus one copy of each row in the left-hand stream_ref for which there was no right-hand row that passed the join condition(extended with nulls on the right). Note a join condition must be specified.
RIGHT [OUTER] JOIN, which produces all data in the qualified Cartesian product by the join condition plus one copy of each row in the right-hand stream_ref for which there was no left-hand row that passed the join condition(extended with nulls on the left). Note a join condition must be specified.
FULL [OUTER] JOIN, which produces all data in the qualified Cartesian product by the join condition, plus one row for each unmatched left-hand row (extended with nulls on the right), plus one row for each unmatched right-hand row (extended with nulls on the left). Note a join condition must be specified.

A join condition can be any of

ON <expression>. The condition passes when the value of the expression is TRUE.
USING(column[, ...]). The specified column(s) is matched.
NATURAL. The common columns of two stream_refs are matched. It is equivalent to USING(common_columns).

# Time Windows

A stream_ref can also have a time window. Currently, we support the following 3 time-window functions:

Tumble( <identifier>, <some_interval>)
HOP( <identifier>, <some_interval>, <some_interval>)
SLIDE( <identifier>, <some_interval>)

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Note that

some_interval represents a period of time. See Intervals.

# Examples

A simple query:

SELECT * FROM my_stream;

Filtering rows:

SELECT temperature, humidity FROM weather WHERE temperature > 10 AND humidity < 75;

Joining streams:

SELECT stream1.temperature, stream2.humidity FROM stream1 JOIN stream2 USING(humidity) WITHIN (INTERVAL '1' HOUR);

Grouping records:

SELECT COUNT(*) FROM TUMBLE(weather, INTERVAL '10' SECOND) GROUP BY cityId;

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