Convert ISO 8601 duration to decimal time sql values such as PT7H30M or PT8H0M, Sql Server

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CREATE TABLE [dbo].[timetracking](
    [qbsql_id] [int] IDENTITY(1,1) NOT NULL,
    [username_id] [int] NULL,
    [TxnDate] [datetime2](0) NULL,
    [Duration] [varchar](50) NULL,
PRIMARY KEY CLUSTERED ([qbsql_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET IDENTITY_INSERT [dbo].[timetracking] ON 
GO

INSERT [dbo].[timetracking] ([qbsql_id], [username_id], [TxnDate], [Duration]) VALUES 
(1, 1, CAST(N'2018-02-02T00:00:00.0000000' AS DateTime2), N'PT8H0M'),
(2, 2, CAST(N'2018-02-01T00:00:00.0000000' AS DateTime2), N'PT7H30M'),
(3, 1, CAST(N'2018-02-01T00:00:00.0000000' AS DateTime2), N'PT1H0M'),
(4, 1, CAST(N'2018-02-01T00:00:00.0000000' AS DateTime2), N'PT4H45M5S')
GO
SET IDENTITY_INSERT [dbo].[timetracking] OFF

;WITH CTE AS
    (
        SELECT  [qbsql_id], 
                [username_id], 
                [TxnDate], 
                [Duration],
                CHARINDEX('P', Duration) As Ppos,
                CHARINDEX('Y', Duration) As Ypos,
                CHARINDEX('M', Duration) As Monpos,
                CHARINDEX('D', Duration) As Dpos,
                CHARINDEX('T', Duration) As Tpos,
                CHARINDEX('H', Duration) As Hpos,
                CHARINDEX('M', Duration, CHARINDEX('T', Duration)) As Minpos,
                CHARINDEX('S', Duration) As Spos
        FROM timetracking
    )

SELECT  [qbsql_id], 
            [username_id], 
            [TxnDate], 
            [Duration],
            0.0 + 
            CASE WHEN Ppos = 1 AND Tpos = 2 THEN
                CASE WHEN Hpos > Tpos THEN
                    ISNULL(CAST(SUBSTRING([Duration], Tpos+1, Hpos - Tpos-1) as float), 0)
                ELSE 
                    0
                END
                + 
                CASE WHEN Minpos > Hpos THEN
                    ISNULL(CAST(SUBSTRING([Duration], Hpos+1, Minpos - Hpos-1) as float), 0) / 60.0
                ELSE 
                    0
                END 
                +
                CASE WHEN Spos > Minpos THEN
                    ISNULL(CAST(SUBSTRING([Duration], Minpos+1, Spos - Minpos-1) as float), 0) / 60.0 /  60.0
                ELSE 
                    0
                END 
            END AS DurationInHours
    FROM CTE

/*
  	qbsql_id	username_id 	TxnDate             	Duration   	DurationInHours
    1       	1           	02.02.2018 00:00:00 	PT8H0M     	8
    2       	2           	01.02.2018 00:00:00 	PT7H30M 	7,5
    3       	1           	01.02.2018 00:00:00 	PT1H0M  	1
*/

View schema
Execution time: 0,03 sec, rows selected: 4, rows affected: 4, absolute service time: 0,16 sec

Convert ISO 8601 duration to decimal time sql values such as PT7H30M o...