SQL Server 2019

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CREATE TABLE dbo.YourTable (Start varchar(6),
                            [End] varchar(6));
INSERT INTO dbo.YourTable
VALUES('161925','161926'),
      ('155225','155226'),
      ('000000','100000'),
      ('170000','170001'),
      ('170109','170110'),
      ('187654','187655'),
      ('176543','176544'),
      ('000000','100000'),
      ('157890','157891'),
      ('187654','187655');
10 rows affected

SELECT STUFF(STUFF(START,5,0,':'),3,0,':') AS StartTime,
       STUFF(STUFF([END],5,0,':'),3,0,':') AS EndTime
FROM dbo.YourTable;


StartTime
EndTime


16:19:25
16:19:26

15:52:25
15:52:26

00:00:00
10:00:00

17:00:00
17:00:01

17:01:09
17:01:10

18:76:54
18:76:55

17:65:43
17:65:44

00:00:00
10:00:00

15:78:90
15:78:91

18:76:54
18:76:55


SELECT TRY_CONVERT(time(0),STUFF(STUFF(START,5,0,':'),3,0,':')) AS StartTime,
       TRY_CONVERT(time(0),STUFF(STUFF([END],5,0,':'),3,0,':')) AS EndTime
FROM dbo.YourTable;


StartTime
EndTime


16:19:25
16:19:26

15:52:25
15:52:26

00:00:00
10:00:00

17:00:00
17:00:01

17:01:09
17:01:10

null
null

null
null

00:00:00
10:00:00

null
null

null
null


DROP TABLE dbo.YourTable;

StartTime	EndTime
16:19:25	16:19:26
15:52:25	15:52:26
00:00:00	10:00:00
17:00:00	17:00:01
17:01:09	17:01:10
18:76:54	18:76:55
17:65:43	17:65:44
00:00:00	10:00:00
15:78:90	15:78:91
18:76:54	18:76:55