先移转，再查询
SELECT case when charindex('12',s) =0 then 0 else length(s)-charindex(reverse('12'),reverse(s)) end AS lastIndexOf, s FROM t;
利用存储过程，一个个找：
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
CREATE FUNCTION [dbo].[lastindexof] (@stringValue as nvarchar(1000), @stringSearch as nvarchar(1000), @startPosition as int = 0)
returns int
AS
BEGIN
DECLARE @lastindex int
SET @lastindex= 0
DECLARE @tempindex int
while (1=1)
begin
SET @tempindex = charindex(@stringSearch, @stringValue, @lastindex + 1)
if (@tempindex = 0)
break
SET @lastindex = @tempindex
end
RETURN(@lastindex)
END