MySQL - Column value depends on values from two different tables I have a MySQL database that contains these tables (* denotes primary key):

TableA:

----------------------------------------------
|  *id  |  time  |  price  |  special_price  |
----------------------------------------------


TableB:

-------------------------------------
|  *table_a_id  |  *date  |  price  |
-------------------------------------


SpecialDate:

--------------------------
|  *date  |  description |
--------------------------


The value of TableB.price column of each entry in TableB will be either TableA.price or TableA.special_price of the corresponding TableA entry, depending on whether TableB.date is a special date or not (whether TableB.date exists in the SpecialDate table).

This design, as it stands, doesn't feel right as every time I'm adding a record to TableB, I have to look up the date from SpecialDate to see if it's a special date or not, then look up the corresponding entry from TableA to get the value for the price column. And every time a record from SpecialDate gets added or updated, I have to go over every entry in TableB to update its price column.

I'm thinking the column TableB.price should be removed and calculated when needed but I'm not sure how to do that using SQL. I have a lot of views that depend on that price column, for example:

TableAFullView:

-----------------------------------------------------------------------------
|  id  |  time  |  count_of_table_b_instances  |  sum_of_table_b_instances  |
-----------------------------------------------------------------------------


Where count_of_table_b_instances is the number of TableB entries that point to this TableA entry and sum_of_table_b_instances is the sum of their prices.

With my original design, this view is simple to implement and it performs well. But if I get rid of the column TableB.price, how would I implement this view? Will the performance take a hit?
With my original design, this view is simple to implement and it performs well. But if I get rid of the column TableB.price, how would I implement this view


create table TableA (
id int,
v_time date,
price int,
special_price int
);

create table TableB (
table_a_id int,
v_date date
);

create table SpecialDate (
v_date date,
description varchar(5)
);

insert into TableA values (1,'2019-09-01', 10, 5);
insert into TableB values (1,'2019-10-01');
insert into TableB values (1,'2019-11-01');

insert into TableA values (2,'2019-12-01', 50, 1);
insert into TableB values (2,'2019-12-02');
insert into TableB values (2,'2019-12-03');
insert into TableB values (2,'2019-11-01');

insert into SpecialDate values('2019-11-01', 'sale');

--your view definition

select a.id,
   a.v_time,
   count(*) as count_of_table_b_instances,
   sum(
      case
     when s.v_date is not null then a.special_price
     else a.price
     end
   ) as sum_of_table_b
 from TableA a
 join TableB b
   on a.id = b.table_a_id
 left join SpecialDate s
  on b.v_date = s.v_date
 group by a.id,
      a.v_time



  Will the performance take a hit?


I can't say for sure because I can't assume you have the right indexes setup to support this query without seeing the DDL of the tables. If you have indexes in place to support the join clauses, or if you don't have much data in these tables, I don't think you would take a performance hit.