创建一个条件SQL选择语句，该语句根据MYSQL中的计数插入

Question

I am trying to develop a statement that counts the sizes of a product and insert a statement base on the amount of that count.

For Example,

I have a shirt that has 3 sizes (S, M, L) For reach size I would need to insert a SQL statement. The insert statement is to insert on Table 2 the menu position.

Table1

ID |  Product | Size
1      Shirt      S  
2      Shirt      M
3      Shirt      L

Table2

ID  |  Dropdown_menu_Position
1        0
2        1
3        2

I know the following query is incorrect but I am struggling with the logic behind it. Any SQL guru can help solve or guide me in the right direction?

INSERT INTO Table2
CASE
WHEN COUNT (SIZE) = 1 THEN
     SELECT NULL, '0' 
WHEN COUNT (SIZE) = 2 THEN
     SELECT NULL, '1' 
WHEN COUNT (SIZE) = 3 THEN
     SELECT NULL, '2' 
ELSE ''
END

I am new at making CASE statement and maybe an IF statement would work better for this, but neither one I know.

Answer 1

IF you are using MySQL 8.0, you can use window functions for this. They can be used to generate a numbering based on a group:

insert into Table2(Id, Dropdown_menu_Position)
select
  t.ID,
  --t.Product,
  --t.Size,
  ROW_NUMBER() over (
    order by s.SortOrder
  ) partition by (t.Product) as Dropdown_menu_Position
from
  Table1 t
  inner join (
    -- Inlined now, but maybe you can make a lookup table for this
    select 'XS' as Size, 0 as SortOrder
    union all select 'S', 1
    union all select 'M', 2
    union all select 'L', 3
    union all select 'XL', 4
    union all select 'XXL', 5
  ) s on s.Size = t.Size

In earlier versions of MySQL, these functions don't exist, and this won't work. However, for this particular case, you should indeed be able to use count to check how many rows there are before the current one. If you have a separate table for the sizes, this query is quite readable, but if not, you would have to embed the lookup table twice in the query. The sample below assumed a separate table:

-- Assuming a lookup table like this:
create table Sizes as
  select 'XS' as Size, 0 as SortOrder
  union all select 'S', 1
  union all select 'M', 2
  union all select 'L', 3
  union all select 'XL', 4
  union all select 'XXL', 5;

-- Select all IDs and how often a smaller size exists for that product
insert into Table2(Id, Dropdown_menu_Position)
select
  t.ID,
  ( select count(*)
    from 
      Table1 t2
      inner join Sizes s2 on s2.Size = t2.Size 
    where
      t2.Product = t.Product and
      s2.SortOrder < s.SortOrder) as Dropdown_menu_Position
from
  Table1 t
  inner join Sizes s on s on s.Size = t.Size

But, maybe you don't need the numbers in Table2 to be consecutive. Maybe it's okay, as long as they have the right order (so M is below S in the menu). This will, of course, depend on your code, but if that is possible, you can simply write this:

insert into Table2(Id, Dropdown_menu_Position)
select
  t.ID,
  s.SortOrder
from
  Table1 t
  inner join (
    -- Inlined now, but maybe you can make a lookup table for this
    select 'XS' as Size, 0 as SortOrder
    union all select 'S', 1
    union all select 'M', 2
    union all select 'L', 3
    union all select 'XL', 4
    union all select 'XXL', 5
  ) s on s.Size = t.Size