Cartesian products of tables that contains same columns

问题

I am really confused about this. I searched a lot of tutorials but i couldnot find a clear answer.

A  B        B  D
1  X        x  5
2  x        y  6
            x  4

I want to cross this two tables.A , B, B,d are attributes.

A  B   B  D
1  X   x  5
2  x   x  5
1  X   y  6
2  x   y  6
1  X   x  4
2  x   x  4

This should be answer normally according to rule of cartesian. Cross all rows. But i am confused about same column B. Same column will seem twice?

回答1:

Some relational query language/algebra relations have ordered column names. There is a way to reference a column by column number. So an attribute name can name more than one column. The Cartesian product operator can take any two relations, because it is not a problem for an attribute name to appear more than once on output (or input).

Some relational query language/algebra Cartesian product operators change attribute names on output. Input relations have names. (The input is ether names of relation variables/constants or relation values and names for them.) Each output column name is a combination of an input column name and an input relation name. The Cartesian product operator can take any two relations. Because columns that appear in different relations with the same attribute name on input will be distinguished by input relation name on output.

Some relational query languages/algebras have both. The Cartesian product operator can take any two relations.

Some relational query language/algebra relations have only one copy of an attribute name. So Cartesian product can only be called when inputs don't share any attribute names. (It is just a special case of NATURAL JOIN and/or EQUIJOIN.)

So you must pay close attention to what kind of relations and operators a relational query language/algebra has.

PS: SQL query language

The FROM clause of a SELECT expression makes a temporary Cartesian product of its tables. Every column name is an input column name prefixed by a given table name or table alias, plus a dot. This is the changes-names verions of Cartesian product above. The dotted names are used in the rest of the SELECT expression. The SELECT clause finally removes the prefixes and dots. So there are no dots in an SQL table value outside a SELECT expression. (The columns are also ordered, and there can be duplicate column names.)

If your SQL table expressions are AB & BD then you just need

SELECT * FROM AB ab CROSS JOIN BD bd

回答2:

Dont you just need the following;

SELECT * FROM TAbleA CROSS JOIN TableB

来源：https://stackoverflow.com/questions/33942237/cartesian-products-of-tables-that-contains-same-columns

标签

relational-algebra

cartesian-product