SQL Server 2017

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declare @Chart table([ChartId] int NOT NULL identity(1,1), [ParentId] int, [Number] varchar(5), 
    [Serial] varchar(5), [AccountName] varchar(200), CurrencyId int, cType char(1),
    [TotalDebit] decimal(18,2), [TotalCredit] decimal(18,2));

insert into @Chart
select null,  '1    ', null, 'CAPITAL & LONG TERM LIABILITIES', 1, 'T', 0, 0 union all
select null, '10   ', null, 'CAPITAL & LONG TERM LIABILITIES', 1, 'T', 0, 0 union all
select null, '101  ', null, 'CAPITAL & LONG TERM LIABILITIES', 1, 'T', 0, 0 union all
select null, '1013 ', null, 'CAPITAL & LONG TERM LIABILITIES', 1, 'T', 0, 0 union all
select null, '1013 ', '00001', 'ShareHolder 1', 1, 'R', 0, 0 union all
select null, '1013 ', '00002', 'ShareHolder 2', 1, 'R', 0, 0 union all
select null, '1013 ', '00003', 'ShareHolder 3', 1, 'R', 0, 0 union all
select null, '4     ', null, 'SUPPLIERS', 1, 'T', 0, 0 union all
select null, '40   ', null, 'SUPPLIERS', 1, 'T', 0, 0 union all
select null, '401  ', null, 'SUPPLIERS', 1, 'T', 0, 0 union all
select null, '4011 ', null, 'SUPPLIERS', 1, 'T', 0, 0 union all
select null, '4011 ', '00101', 'Supplier 1', 1, 'R', 0, 0 union all
select null, '4011 ', '00102', 'Supplier 1', 2, 'R', 0, 0 union all
select null, '4011 ', '00103', 'Supplier 2', 1, 'R', 0, 0 union all
select null, '4011 ', '00104', 'Supplier 3', 2, 'R', 0, 0 union all
select null, '4011 ', '00105', 'Supplier 3', 1, 'R', 0, 0 union all
select null, '4011 ', '00105', 'Supplier 3', 3, 'R', 0, 0 union all
select null, '41   ', null, 'CUSTOMERS', 1, 'T', 0, 0 union all
select null, '411  ', null, 'CUSTOMERS', 1, 'T', 0, 0 union all
select null, '4111 ', null, 'CUSTOMERS', 1, 'T', 0, 0 union all
select null, '4111 ', '00101', 'Customer 1', 1, 'R', 0, 0 union all
select null, '4111 ', '00102', 'Customer 2', 1, 'R', 0, 0 union all
select null, '4111 ', '00103', 'Customer 2', 2, 'R', 0, 0 union all
select null, '4111 ', '00104', 'Customer 3', 2, 'R', 0, 0 union all
select null, '4111 ', '00105', 'Customer 4', 2, 'R', 0, 0 union all
select null, '4111 ', '00106', 'Customer 4', 3, 'R', 0, 0 union all
select null, '4111 ', '00107', 'Customer 5', 3, 'R', 0, 0;

SELECT
    ChartId,
    [Number],
    Serial,
    FIRST_VALUE([ChartId]) OVER (PARTITION BY [Number] ORDER BY [Number], Serial)
FROM
    @Chart;

/*
UPDATE c0
SET    ParentId = CASE WHEN c0.Serial IS NULL
                  THEN (SELECT TOP 1 c1.ChartId
                        FROM   @Chart c1
                        WHERE  c1.[Number] < c0.[Number]
                               AND c1.Serial IS NULL
                               ORDER BY c1.[Number] DESC)
                  ELSE (SELECT TOP 1 c2.ChartId 
                        FROM   @Chart c2
                        WHERE  c2.[Number] = c0.[Number] 
                        AND    c2.Serial IS NULL)
                  END
FROM   @Chart c0; 
*/

;WITH x AS
(
    SELECT 
        c1.ChartId, 
        CASE WHEN c1.Serial IS NULL
             THEN LAG(c1.ChartId) OVER (ORDER BY c1.[Number], c1.Serial)
             ELSE FIRST_VALUE([ChartId]) OVER (PARTITION BY [Number] ORDER BY [Number], Serial)
             END AS ParentID
    FROM 
        @Chart c1
)
UPDATE c1
SET    ParentId = x.ParentId
FROM   @Chart c1
JOIN   x 
       ON x.ChartId = c1.ChartId;

SELECT * FROM @Chart;

ChartId	Number	Serial	(No column name)
1	1	null	1
2	10	null	2
3	101	null	3
4	1013	null	4
5	1013	00001	4
6	1013	00002	4
7	1013	00003	4
8	4	null	8
9	40	null	9
10	401	null	10
11	4011	null	11
12	4011	00101	11
13	4011	00102	11
14	4011	00103	11
15	4011	00104	11
16	4011	00105	11
17	4011	00105	11
18	41	null	18
19	411	null	19
20	4111	null	20
21	4111	00101	20
22	4111	00102	20
23	4111	00103	20
24	4111	00104	20
25	4111	00105	20
26	4111	00106	20
27	4111	00107	20

ChartId	ParentId	Number	Serial	AccountName	CurrencyId	cType
1	null	1	null	CAPITAL & LONG TERM LIABILITIES	1	T
2	1	10	null	CAPITAL & LONG TERM LIABILITIES	1	T
3	2	101	null	CAPITAL & LONG TERM LIABILITIES	1	T
4	3	1013	null	CAPITAL & LONG TERM LIABILITIES	1	T
5	4	1013	00001	ShareHolder 1	1	R
6	4	1013	00002	ShareHolder 2	1	R
7	4	1013	00003	ShareHolder 3	1	R
8	7	4	null	SUPPLIERS	1	T
9	8	40	null	SUPPLIERS	1	T
10	9	401	null	SUPPLIERS	1	T
11	10	4011	null	SUPPLIERS	1	T
12	11	4011	00101	Supplier 1	1	R
13	11	4011	00102	Supplier 1	2	R
14	11	4011	00103	Supplier 2	1	R
15	11	4011	00104	Supplier 3	2	R
16	11	4011	00105	Supplier 3	1	R
17	11	4011	00105	Supplier 3	3	R
18	17	41	null	CUSTOMERS	1	T
19	18	411	null	CUSTOMERS	1	T
20	19	4111	null	CUSTOMERS	1	T
21	20	4111	00101	Customer 1	1	R
22	20	4111	00102	Customer 2	1	R
23	20	4111	00103	Customer 2	2	R
24	20	4111	00104	Customer 3	2	R
25	20	4111	00105	Customer 4	2	R
26	20	4111	00106	Customer 4	3	R
27	20	4111	00107	Customer 5	3	R