Existem várias maneiras de selecionar de forma randômica um registro no banco de dados. A sintaxe varia de banco para banco.

MySQL:

SELECT column FROM table
ORDER BY RAND()
LIMIT 1

PostgreSQL:

SELECT column FROM table
ORDER BY RANDOM()
LIMIT 1

Microsoft SQL Server:

SELECT TOP 1 column FROM table
ORDER BY NEWID()

IBM DB2:

SELECT column, RAND() as IDX
FROM table
ORDER BY IDX FETCH FIRST 1 ROWS ONLY

Oracle:

SELECT column FROM
( SELECT column FROM table ORDER BY dbms_random.value )
WHERE rownum = 1

Referências:

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