Oracle 21c

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CREATE TABLE cliente (id PRIMARY KEY) AS
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL UNION ALL
SELECT 4 FROM DUAL UNION ALL
SELECT 5 FROM DUAL UNION ALL
SELECT 6 FROM DUAL;
6 rows affected

CREATE TABLE exames (nu_ordem PRIMARY KEY, id_cliente) AS
SELECT  1, 1 FROM DUAL UNION ALL
SELECT  2, 5 FROM DUAL UNION ALL
SELECT  3, 5 FROM DUAL UNION ALL
SELECT  4, 2 FROM DUAL UNION ALL
SELECT  5, 6 FROM DUAL UNION ALL
SELECT  6, 1 FROM DUAL UNION ALL
SELECT  7, 1 FROM DUAL UNION ALL
SELECT  8, 4 FROM DUAL UNION ALL
SELECT  9, 5 FROM DUAL UNION ALL
SELECT 10, 3 FROM DUAL UNION ALL
SELECT 11, 6 FROM DUAL UNION ALL
SELECT 12, 2 FROM DUAL UNION ALL
SELECT 13, 1 FROM DUAL;
13 rows affected

SELECT AVG(COUNT(e.nu_ordem)) AS avg_exames_by_client
FROM   cliente c
       LEFT OUTER JOIN exames e
       ON (c.id = e.id_cliente)
GROUP BY c.id;


AVG_EXAMES_BY_CLIENT


2.16666666666666666666666666666666666667


SELECT (SELECT COUNT(*) FROM exames) / (SELECT COUNT(*) FROM cliente)
         AS avg_exames_by_client
FROM   DUAL;


AVG_EXAMES_BY_CLIENT


2.16666666666666666666666666666666666667


INSERT INTO cliente (id)
SELECT 7 FROM DUAL UNION ALL
SELECT 8 FROM DUAL
2 rows affected

SELECT AVG(COUNT(e.nu_ordem)) AS avg_exames_by_client
FROM   cliente c
       LEFT OUTER JOIN exames e
       ON (c.id = e.id_cliente)
GROUP BY c.id;


AVG_EXAMES_BY_CLIENT


1.625


SELECT (SELECT COUNT(*) FROM exames) / (SELECT COUNT(*) FROM cliente)
         AS avg_exames_by_client
FROM   DUAL;


AVG_EXAMES_BY_CLIENT


1.625