MERGE Statement

Syntax 

MERGE stream_expression_1, stream_expression_2
  USING field_identifier_m [GROUP BY field_identifier_g]
  [ERROR INTO stream_identifier];

Substitutable Fields

stream_expression_n

Either the unique identifier (name) of a stream or a parenthesized SELECT statement. If a stream has the desired output schema (including field names, types, and order), the stream identifier can be used directly. If it is necessary to rename, reorder, or change the type of a field, then the SELECT statement must be used.

field_identifier_m

The tuple field used to merge the incoming streams into an ordered output stream.

field_identifier_g

The tuple field used to group tuples prior to merging.

ERROR INTO Clause

You can append an ERROR INTO clause just before the closing semicolon. The StreamSQL ERROR INTO clause is analogous to the Enable Error Output Port check box for operators and adapters in EventFlow applications.

Use ERROR INTO with the name of a stream, which must already exist. This sets up an Error Port for this operator, which is much like a local catch mechanism for errors from this operator.

See Using Error Ports and Error Streams for a discussion of StreamBase error handling mechanisms.

Discussion

MERGE takes two input streams with compatible schemas and produces one output stream with all the tuples from the original streams. MERGE has a strict guarantee of output order based on some user-specified expression over all tuples. Incoming tuples are pre-sorted in increasing order of some tuple field.

Optionally, tuples can also be grouped by one or more input fields. Any groups that you define are ordered independently when they are merged. MERGE stores arriving tuples in a buffer for each input port. It emits tuples when a new tuple's value (based on the field that was selected to merge on) is greater than or equal to the oldest tuple in the other buffer. If the group option has been selected, the tuples must also evaluate to the same group.

The stream_expression_n entries can be either the names of streams with identical tuple structure or a parenthesized SELECT statement. In a parenthesized SELECT statement, use the target list and AS keyword to enable merging of streams with incompatible tuple structure. The USING keyword identifies the merge field; GROUP BY identifies the grouping specification. These cannot be the same tuple fields.

MERGE generates a stream and can be used anywhere a stream expression is acceptable. As an alternative, the output could be captured in a stream, as illustrated in the following code fragments. 

CREATE [OUTPUT] STREAM stream_identifier;
MERGE ... INTO stream_identifier;

CREATE [OUTPUT] STREAM stream_identifier AS
  MERGE ...;

Or, for an OUTPUT STREAM 

MERGE ... => CREATE OUTPUT STREAM stream_identifier;

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