DECLARE @GLmaster TABLE
(
	Glcode 	varchar(10),
	Gldesc	varchar(20)
)
INSERT INTO @GLmaster
SELECT	'GL01', 'Plant & Machinery'	UNION ALL
SELECT	'GL02', 'Land & Building'	UNION ALL
SELECT	'GL03', 'Staff Welfare'		UNION ALL
SELECT	'GL04', 'Conveyance'

DECLARE @Slmaster TABLE
(
	Glcode 	varchar(10),
	Slcode 	varchar(10),
	Sldesc	varchar(20)
)
INSERT INTO @Slmaster
SELECT	'GL03', 'SL01', 'Tea Expenses'		UNION ALL
SELECT	'GL03', 'SL02', 'Food Expenses'		UNION ALL
SELECT	'GL03', 'SL03', 'Fitness Expenses'	UNION ALL
SELECT	'GL04', 'SL04', 'Bus fare'		UNION ALL
SELECT	'GL04', 'SL05', 'Train fare'

DECLARE @Trans TABLE
(
	Refno 	varchar(10),
	Glcode 	varchar(10),
	Slcode 	varchar(10),
	Amount	int
)
INSERT INTO @Trans
SELECT	'RF01', 'GL01', NULL,   1000	UNION ALL
SELECT	'RF02', 'GL04', 'SL04', 400	UNION ALL
SELECT	'RF03', 'GL02', NULL,   2000	UNION ALL
SELECT	'RF04', 'GL03', 'SL02', 200

SELECT	t.Refno, t.Glcode, t.Slcode, t.Amount, [DESC] = coalesce(g.Gldesc, s.Sldesc)
FROM	@Trans t 
	left JOIN @GLmaster g
	ON	t.Glcode	= g.Glcode
	AND	t.Slcode	IS NULL
	left JOIN @Slmaster s
	ON	t.Glcode	= s.Glcode
	AND	t.Slcode	= s.Slcode

/*
Refno      Glcode     Slcode     Amount      DESC                 
---------- ---------- ---------- ----------- -------------------- 
RF01       GL01       NULL              1000 Plant & Machinery
RF02       GL04       SL04               400 Bus fare
RF03       GL02       NULL              2000 Land & Building
RF04       GL03       SL02               200 Food Expenses
*/