SQL Server 2017

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CREATE TABLE #Demo
(
    [Tag Type] char(1) NOT NULL,
    [Parent Tag Type] char(1) NULL,
    [Tag Area No] integer NOT NULL,
    [Tag seq No] integer NOT NULL,
    [Tag Suffix] char(1) NULL,

    [Tag] AS CONCAT([Tag Type], [Parent Tag Type], [Tag Area No], [Tag seq No], [Tag Suffix])
);
INSERT #Demo
    ([Tag Type], [Parent Tag Type], [Tag Area No], [Tag seq No], [Tag Suffix])
VALUES
    ('T', NULL, 1234, 1, NULL),
    ('T', 'P', 5678, 1, 'S');
2 rows affected

SELECT * FROM #Demo AS D;


Tag Type
Parent Tag Type
Tag Area No
Tag seq No
Tag Suffix
Tag


T
null
1234
1
null
T12341

T
P
5678
1
S
TP56781S