Getting Previous Record From Another Table

Hello

just wondering, if I can get help with a table (Table A) , where there are few dates and numbers, I have to relate this table to another table (Table B) , where I have whole year working dates

I want to write an expression while making dataset, that if date in table B matches table A, it just get the number from the next column from table A, if it does not matches - it should go back to the date (in table A) - where there is a number.

Like table B has working dates for march, but there are only two dates in table A , for 3/2 (and number is 300) and 3/20 (number 200). So In dataset I want all dates from table B and from 3/2 to 3/19 300 in number column and 200 on 3/20.

Thanks in advance for your help