mysql - Constraint on a column based on another column

Question

I have a table "Table" that contains an ID, Col1 and Col2, Col3. Col2 can either be 0 or 1. I want Col2 in rows where Col1 has the same value to be the same. Ex

I want something like this

+----+-------+------+-----------+
| ID | Col1  | Col2 |   Col3    |
+----+-------+------+-----------+
|  1 | "One" |    0 | "Yeah"    |
|  2 | "One" |    0 | "Meh"     |
|  3 | "One" |    0 | "Why Not" |
|  4 | "Two" |    1 | "Huh"!    |
+----+-------+------+-----------+

And not

+----+-------+------+-----------+
| ID | Col1  | Col2 |   Col3    |
+----+-------+------+-----------+
|  1 | "One" |    0 | "Yeah"    |
|  2 | "One" |    0 | "Meh"     |
|  3 | "One" |    1 | "Why Not" | (Normally it must be 0 or line 1 and 2 
|  4 | "Two" |    1 | "Huh"!    | Must be "1" )
+----+-------+------+-----------+

Answer 1

Even if MySQL supported check constraints, you wouldn't do this with check constraints. I think the best way is using foreign key constraints.

You need a second table with the valid col1/col2 values:

create table Col1Col2 as (
    col1 varchar(255) not null primary key,
    col2 int not null,
    unique (col1, col2)  -- this is not strictly necessary see below
);

Then your table would be:

create table t as (
    id int auto_increment primary key,
    col1 varchar(255) not null,
    col2 int not null
    col3 int,
    constraint fk_t_col1_col2 foreign key (col1, col2) references col1col2(col1, col2)
);

However, I wouldn't even store col2 in t. Instead remove it from t and just look up the value in col1col2.

Your basic problem is that you are not storing the data in a relational format, because this requirement suggests that you have another entity.