Get the latest row in each group.
Select one ordered row per key with DISTINCT ON.
CREATE TABLE readings (sensor TEXT, sequence INTEGER, value INTEGER);
INSERT INTO readings VALUES ('A',1,10),('A',2,12),('B',1,7);
SELECT DISTINCT ON (sensor) sensor, value
FROM readings ORDER BY sensor, sequence DESC;Expected output EXAMPLE
A|12 B|7
Details
Requires PostgreSQL 18. Run in an empty scratch schema using psql or a SQL client. Result rows below use | between columns, without headers; client display formatting may differ. DISTINCT ON must match the leftmost ORDER BY expressions. Add a tiebreaker if sequence is not unique within a sensor. Each example includes its own data and should be run in a fresh scratch schema.
Tested on Ubuntu 24.04.5 LTS · · PostgreSQL 18.6 (Debian 18.6-1.pgdg13+2) · libc6 2.39-0ubuntu8.9; psycopg 3.2.10; pymssql 2.3.8
Test inputs, results and environment
The exact published code ran in a dedicated Linux test container with prepared inputs. Snippet process uid 65534 with a read-only root filesystem and temporary working files. Database services use a private internal Docker network without published ports. Fresh scratch schema/database for each case; test database administrator credentials are confined to these disposable services. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.
- Architecture / host kernel
- x86_64 / 6.18.54.1-microsoft-standard-WSL2
- Snippet identity / locale
- uid=65534, gid=65534 (unprivileged process) / C.UTF-8
- Interpreter invocation
Execute the exact SQL in a fresh database/schema; serialize result rows with | separators- Container image ID
sha256:ec91410a8f6ec3e985830b5e3a0560306d36f8c1c5ccd91a6699abe707123fa3- Code SHA-256
e6d1694b2fe3f18632084fab3d562cc69da7e4a9d96511ddd8b3c6b15a8d4485- Dependencies
- libc6 2.39-0ubuntu8.9; psycopg 3.2.10; pymssql 2.3.8
Test input setup
These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.
# No input files required.
Observed standard output
A|12 B|7
Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.