COUNT function - Amazon Quantum Ledger Database (Amazon QLDB)

COUNT function

Returns the number of documents that are defined by the expression. The COUNT 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