MariaDB 10.5

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CREATE TABLE dossier
AS
SELECT 1 origine_client_id, 2 annee_imputation  UNION ALL
SELECT NULL origine_client_id, 1 annee_imputation UNION ALL
SELECT NULL, NULL UNION ALL
SELECT 1 origine_client_id, NULL annee_imputation;

Records: 4  Duplicates: 0  Warnings: 0

SELECT COUNT(DISTINCT origine_client_id,  annee_imputation) FROM dossier d1;

COUNT(DISTINCT origine_client_id,  annee_imputation)

1

SELECT COUNT(DISTINCT d2.origine_client_id,  d2.annee_imputation) FROM (SELECT origine_client_id,  annee_imputation FROM dossier) as d2;

COUNT(DISTINCT d2.origine_client_id,  d2.annee_imputation)

1

SELECT COUNT(*) FROM (SELECT DISTINCT origine_client_id,  annee_imputation FROM dossier) as d3;

COUNT(*)

4