MySQL and PostgreSQL Row Subqueries operator - mysql

Question

I have been little bit confused while reading MySQL documentation MySQL documentation
A row subquery is a subquery variant that returns a single row and can thus return more than one column value. Legal operators for row subquery comparisons are:
= > < >= <= <> != <=>
For "=" documentation provides good explanation:
SELECT * FROM t1 WHERE (column1,column2) = (1,1);
is same as:
SELECT * FROM t1 WHERE column1 = 1 AND column2 = 1;
But how do the ">" or "<" can compare two rows? What is "plain" variant for this operator?
create table example(a integer,b integer);
insert into example values (1,1);
insert into example values (1,2);
insert into example values (2,1);
select * from example where (a,b) > (1,1)
a | b
-----
1 | 2
2 | 1
Playground: http://www.sqlfiddle.com/#!9/88641/2
p.s.:
PostgreSQL have same behaviour.
Oracle fail with error "ORA-01796: this operator cannot be used with lists "

Answer 1

MySQL and PostgreSQL has same behaviour, let's read postgres documentation
The = and <> cases work slightly differently from the others. Two rows are considered equal if all their corresponding members are non-null and equal; the rows are unequal if any corresponding members are non-null and unequal; otherwise the result of the row comparison is unknown (null).
For the <, <=, > and >= cases, the row elements are compared left-to-right, stopping as soon as an unequal or null pair of elements is found. If either of this pair of elements is null, the result of the row comparison is unknown (null); otherwise comparison of this pair of elements determines the result. For example, ROW(1,2,NULL) < ROW(1,3,0) yields true, not null, because the third pair of elements are not considered.
So (a,b) = (c,d) evaluated as
a = c and b = d
But (a,b,) < (c,d) evaluated as
a < b or (a=c and b < c)
It's look very strange. And I have no guess how will be evaluated comparation of rows with 3 of more attributes;