Show the total database size

Oracle

SELECT ROUND(SUM(bytes)/1024/1024/1024, 2) AS total_gb
FROM dba_data_files;

For used segments only, sum dba_segments. Add dba_temp_files for TEMP.

PostgreSQL

SELECT pg_size_pretty(pg_database_size(current_database()));

For all databases: SELECT datname, pg_size_pretty(pg_database_size(datname)) FROM pg_database ORDER BY 2 DESC;

MySQL

SELECT table_schema AS db,
       ROUND(SUM(data_length + index_length)/1024/1024, 1) AS mb
FROM information_schema.tables
GROUP BY table_schema
ORDER BY mb DESC;

For InnoDB this value is an estimate; for the exact file size check ibdata/the files at the OS level.

SQL Server

EXEC sp_spaceused;
-- All databases:
SELECT DB_NAME(database_id) AS db,
       CAST(SUM(size) * 8.0 / 1024 AS DECIMAL(10,1)) AS mb
FROM sys.master_files
GROUP BY database_id;

The size column is in 8 KB pages; * 8 / 1024 converts it to MB.

MongoDB

db.stats(1024*1024)
// returns the dataSize, storageSize, indexSize fields in MB

storageSize is the compressed on-disk size; dataSize is the uncompressed logical size.

Redis

INFO memory
# the used_memory_human line gives the total size in memory
DBSIZE  # number of keys

Redis is memory-based; 'size' means RAM usage. The RDB/AOF file size is the persisted state on disk.