组上的行编号不正确

Question

and thanks for helping me learn mysql.

I'm trying to dynamically-generate row numbers that only increment when there is a new record in a group (for a given column). My code sort of works - it changes when it should, but the values are all wrong.

I have a table called fruits with only one column ( varchar ) called preference . It has nine (9) records in this order:

Apples
Pears
Apples
Oranges
Bananas
Pears
Oranges
Apples
Bananas

Its the same challenge (the t-index ) posed in Increment Row Number on Group but that solution used (I think) a new mysql Window function that is only available (I think) in mysql v8 , which isn't supported by my web host (as CPanel doesn't yet support).

Here is my code...

SET @row_num=1,@currFruit='';

SELECT @row_num=CASE WHEN fruit.preference=@currFruit 
                    THEN @row_num 
                    ELSE @row_num:=@row_num+1 END AS 'RowNum', 
    preference, @currFruit:=fruit.preference    
FROM fruit
ORDER BY fruit.preference;

The results show the dynamically-generated row numbers changing when it should, but the initial value is wrong, it shouldn't reset each group, they don't keep incrementing (apples should be 1, 2, 3), (bananas should be 1, 2), (Oranges 1,2), (Pears 1,2)

Here is the incorrect output ....

@RowNum  preference         @currFruit:=fruit.preference
0        Apples             Apples
1        Apples             Apples
1        Apples             Apples
0        Bananas            Bananas
1        Bananas            Bananas
0        Oranges            Oranges
1        Oranges            Oranges
0        Pears              Pears
1        Pears              Pears

Answer 1

The = in @row_num = CASE ... is comparing @row_num to the result of the CASE expression, which returns 1 for true and 0 for false. You should be assigning with := there.

You also have the consequences of the CASE wrong. When it's the same as @currFruit you should add 1, otherwise you should set it back to 1.

SELECT @row_num := CASE WHEN fruit.preference=@currFruit 
                    THEN @row_num + 1
                    ELSE 1 END AS 'RowNum', 
    preference, @currFruit:=fruit.preference    
FROM fruit
ORDER BY fruit.preference

DEMO