COUNT function in Amazon QLDB
In Amazon QLDB, use the COUNT function to return the number of documents
that are defined by the given expression. This function has two variations:
-
COUNT(*)– Counts all of the documents in the target table whether or not they include null or missing values. -
COUNT(expression)– Computes the number of documents with non-null values in a specific, existing field or expression.
Warning
The COUNT function is not optimized, so we don't recommend using it
without an indexed lookup. When you run a query in QLDB without an indexed lookup,
it invokes a full table scan. This can cause performance problems on large tables,
including concurrency conflicts and transaction timeouts.
To avoid table scans, you must run statements with a WHERE predicate
clause using an equality operator (= or
IN) on an indexed field or a document ID. For more information, see
Optimizing query performance.
Syntax
COUNT ( * |expression)
Arguments
expression-
The field name or expression that the function operates on. This parameter can be any of the supported Data types.
Return type
int
Examples
SELECT COUNT(*) FROM VehicleRegistration r WHERE r.LicensePlateNumber = 'CA762X' -- 1 SELECT COUNT(r.VIN) FROM Vehicle r WHERE r.VIN = '1N4AL11D75C109151' -- 1 SELECT COUNT(a) FROM << { 'a' : 1 }, { 'a': 2 }, { 'a': 3 } >> -- 3
Related functions
