What does it mean to say that "the where clause of select statements can perform >, >=, <, <= operations on one column only"?

Question

According to the Wikipedia article on Google App Engine:

The where clause of select statements can perform >, >=, <, <= operations on one column only. Therefore, only simple where clauses can be constructed.

What does this mean?

Answer 1

Sounds to me like you can't compare two columns to each other: WHERE Column1 > Column2 wouldn't work for example.

Answer 2

Google explains it in their article on querying the App Engine datastore. Look for the bulleted list titled, "The filter operator can be any of the following:"

Answer 3

See this document:

For example, this query is allowed:

select from Person where birthYear >= minBirthYearParam
                      && birthYear <= maxBirthYearParam

However, this query is not allowed, because it uses inequality filters on two different properties in the same query:

select from Person where birthYear >= minBirthYearParam
                      && height >= minHeightParam   // ERROR