Multi Select Queries

[jeffgbutler]

A multi-select query is a special case of a union select statement. The difference is that it allows "order by" and
paging clauses to be applied to the nested queries.

Example Java code:

SelectStatementProvider selectStatement = multiSelect(
        select(id, firstName, lastName, birthDate, employed, occupation, addressId)
                .from(person)
                .where(id, isLessThanOrEqualTo(2))
                .orderBy(id)
                .limit(1)
).union(
        select(id, firstName, lastName, birthDate, employed, occupation, addressId)
                .from(person)
                .where(id, isGreaterThanOrEqualTo(4))
                .orderBy(id.descending())
                .limit(1)
)
.orderBy(id)
.limit(2)
.build()
.render(RenderingStrategies.MYBATIS3);

Example Kotlin code:

val selectStatement = multiSelect {
    selectDistinct(id, firstName, lastName, birthDate, employed, occupation, addressId) {
        from(person, "p1")
        where { id isLessThanOrEqualTo 2 }
        orderBy(id)
        limit(1)
    }
    union {
        select(id, firstName, lastName, birthDate, employed, occupation, addressId) {
            from(person, "p2")
            where { id isGreaterThanOrEqualTo 4 }
            orderBy(id.descending())
            limit(1)
        }
    }
    orderBy(id)
    limit(2)
}

Resolves #586

[coveralls]

Coverage: 100.0%. Remained the same when pulling 9b9c04a on jeffgbutler:multi-select-union into ba8a43a on mybatis:master.