select number of children, MySQL

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#MySQL 5.7.12
#please drop objects you've created at the end of the script 
#or check for their existance before creating
#'\\' is a delimiter

drop table if exists yourTable;

CREATE TABLE yourTable
    (`column1` varchar(8), `column2` varchar(9), `column3` varchar(8))
;
    
INSERT INTO yourTable
    (`column1`, `column2`, `column3`)
VALUES
    ('A100', NULL, NULL),
    ('A101', 'B200', 'C300'),
    ('A101', 'B200', 'C301'),
    ('A101', 'B200', 'C302'),
    ('A101', 'B201', 'C310'),
    ('A102', 'B209', NULL),
    ('A103', 'B211', NULL),
    ('A103', 'B213', NULL),
    ('A103', 'B214', NULL),
    ('A104', 'B240', NULL)
;

SELECT DISTINCT a.Column1 AS Parent, a.Column2 AS Child
FROM yourTable AS a
JOIN (SELECT Column1, COUNT(DISTINCT Column2) AS children
      FROM yourTable
      GROUP BY Column1) AS b
ON a.Column1 = b.Column1
WHERE children = 1;

drop table yourTable;


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