MariaDB 10.6

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CREATE TABLE `towns` ( `id` INT NOT NULL AUTO_INCREMENT , 
                      `town` VARCHAR(50) NOT NULL , 
                      `postcode` VARCHAR(10) NOT NULL , PRIMARY KEY (`id`)
                     ); 
 
CREATE TABLE `peoples` (
    `id` INT NOT NULL AUTO_INCREMENT primary key,
    `names` VARCHAR(50) NULL,
    `postcode` VARCHAR(10) NULL
);
INSERT INTO `towns` (town, postcode) VALUES ('Hudderfield', 'HD11 4ER'), 
 ('Manchester','MN14 3JE'), 
('Macklesfield','MK17 9FL'), 
('Edinburgh','ED5  3MJ'), 
('Liverpool','LV9  8XT');

Records: 5  Duplicates: 0  Warnings: 0


INSERT INTO `peoples` (names, postcode) VALUES ('Jimmy Saville','HD11 4ER'),
('Jason Bomb','IP14 8FK'),
(' Micky Mouse','MK17 9FL'),
(' Bobby Dillian','ED5  3MJ'),
('Lenny Davies','ED5  3MJ');

Records: 5  Duplicates: 0  Warnings: 0


SELECT t.id, t.town, p.names 
FROM towns t
LEFT JOIN (
    SELECT names, postcode
    FROM (
       SELECT names, postcode
            , count(1) over (partition by postcode) as cnt
       FROM peoples
    ) as x
    WHERE cnt = 1
) p
    USING (postcode);


id
town
names


1
Hudderfield
Jimmy Saville

2
Manchester
null

3
Macklesfield
 Micky Mouse

4
Edinburgh
null

5
Liverpool
null

id	town	names
1	Hudderfield	Jimmy Saville
2	Manchester	null
3	Macklesfield	Micky Mouse
4	Edinburgh	null
5	Liverpool	null