2. Queries

Queries are the output of a QL program: they evaluate to sets of results. Indeed, we often refer to the whole QL program as a query.

There are two kinds of queries. For a given query module, the queries in that module are:
  • The select clause, if any, defined in that module.
  • Any query predicates in that module’s predicate namespace. That is, they can be defined in the module itself, or imported from a different module.

2.1. Select clauses

When writing a query module, you can include a select clause (usually at the end of the file) of the following form:

from /* ... variable declarations ... */
where /* ... logical formula ... */
select /* ... expressions ... */

The from and where parts are optional.

Apart from the expressions described in Expressions, you can also include:
  • The as keyword, followed by a name. This gives a “label” to a column of results, and allows you to use them in subsequent select expressions.
  • The order by keywords, followed by the name of a result column, and optionally the keyword asc or desc. This determines the order in which to display the results.

For example:

from int x, int y
where x = 3 and y in [0 .. 2]
select x, y, x * y as product, "product: " + product

This select clause returns the following results:

x y product  
3 0 0 product: 0
3 1 3 product: 3
3 2 6 product: 6

You could also add order by y desc at the end of the select clause. Now the results are ordered according to the values in the y column, in descending order:

x y product  
3 2 6 product: 6
3 1 3 product: 3
3 0 0 product: 0

2.2. Query predicates

A query predicate is a non-member predicate with a query annotation. It returns all the tuples that the predicate evaluates to.

For example:

query int getProduct(int x, int y) {
  x = 3 and
  y in [0 .. 2] and
  result = x * y

This predicates returns the following results:

x y result
3 0 0
3 1 3
3 2 6

A benefit of writing a query predicate instead of a select clause is that you can call the predicate in other parts of the code too. In contrast, the select clause is like an anonymous predicate, so you can’t call it later.

It can also be helpful to add a query annotation to a predicate while you debug code. That way you can explicitly see the set of tuples that the predicate evaluates to.