Querying data

As we mentioned in the previous section, database sets and maps are two types of collections that allows the organization of multiple instances of data in the database. We can query a set of values using the following operators:

== expr: equals expr
!= expr: not equals expr
< expr:  lesser than expr
<= expr: lesser than or equals expr
> expr:  greater than expr
>= expr: greater than or equals expr
in expr: "belongs to" expr, where expr is a list
q1 or q2: satisfy query q1 or q2
q1 and q2: satisfy both queries, q1 and q2
not q: does not satisfy q
{f1 q1, f2 q2, ...}: the database field f1 satisfies q1, field f2 satisfies q2 etc.

Furthermore, we can specify some querying options as follows:

• skip n: Here expr should be an expression of type int and it skip the first n results.

• limit n: Here expr should be an expression of type int, returns a maximum of n results.

• order fld (, fld)+: Here fld specifies an order. fld can be a single identifier or a list of identifiers...