MySQL enforce uniqueness across two columns - mysql

Question

I'm trying to achieve something in MySQL that I have not heard is possible before (maybe it is, researching didn't help me much).
What I'm trying to do is enforce uniqueness in MySQL across two columns. What I mean by this is not setting UNIQUE(column1,column2) but the following two conditions:
If a value exists in column1, it cannot be repeated in column1 (same as setting UNIQUE(column1)).
If a value exists in either column, it cannot exist in the other column.
Hence, for the data set {column1,column2}, if {1,2}, {3,4}, {5,6} are data already present, then neither of the two columns can have any of the above data items for new data,i.e. new data item {x,y} where x=NOT{column1} AND y=NOT{column2} AND x!=y
Is this possible? Please help me out here. Thank you.

Answer 1

This might be an overkill, but you can store column1 and column2 in a separate table.
Let's say your table is
create table items (
id int primary key,
column1 int,
column2 int
);
with data:
id | column1 | column2
---|---------|--------
1 | 1 | 2
1 | 3 | 4
1 | 5 | 6
You can change your schema to
create table items (
id int primary key
);
create table item_columns (
item_id int,
position int,
val int,
primary key (item_id, position),
unique key (val),
foreign key (item_id) references items(id)
);
with data:
item_id | position | val
--------|----------|----
1 | 1 | 1
1 | 2 | 2
2 | 1 | 3
2 | 2 | 4
3 | 1 | 5
3 | 2 | 6
You can simulate the old schema with
select i.id, c1.val as column1, c2.val as column2
from items i
left join item_columns c1
on c1.item_id = i.id
and c1.position = 1
left join item_columns c2
on c2.item_id = i.id
and c2.position = 2
You can use it in a view if you like.
Demo: http://rextester.com/PPBT42478
To guaranty the integrity for the position column, you can make it a foreign key to a positions table, which will only contain the values 1 and 2. You could also use ENUM('1', '2'), but ENUM always allow an empty string as a value.