We are going to construct some programs that retrieve data from the database and stream it back, mapping to Scala types on the way.
We will be playing with the country table that has the following structure:
CREATE TABLE country( code character (3) NOT NULL, name text NOT NULL, population integer NOT NULL, gnp numeric (10, 2) )
For the exercises, the country table will contain:
code name population gnp "DEU" "Germany" 82164700 2133367.00 "ESP" "Spain" 39441700 null "FRA" "France" 59225700 1424285.00 "GBR" "United Kingdom" 59623400 1378330.00 "USA" "United States of America" 278357000 8510700.00
As we commented in the previous section, the
sql string interpolator allows us to create a
query to select data from the database.
sql"select name from country".query[String] defines a
is a one-column query that maps each returned row to a String.
Once we generate this query, we could use several convenience methods to stream the results:
.list, which accumulates the results to a
List, in this case yielding a
.vector, which accumulates to a
.to[Coll], which accumulates to a type
Coll, given an implicit
CanBuildFrom. This works with Scala standard library collections.
.accumulate[M[_]: MonadPlus]which accumulates to a universally quantified monoid
M. This works with many scalaz collections, as well as standard library collections with
.uniquewhich returns a single value, raising an exception if there is not exactly one row returned.
.optionwhich returns an Option, raising an exception if there is more than one row returned.
.nelwhich returns an
NonEmptyList, raising an exception if there are no rows returned.
Query0for more information on these and other methods.
We can use the
unique method if we expect the query to return only one row
val countryName = sql"select name from country where code = 'ESP'" .query[String] .unique .transact(xa) .run countryName should be(res0)
If we are not sure if the record exists, we can use the
val maybeCountryName = sql"select name from country where code = 'ITA'" .query[String] .option .transact(xa) .run maybeCountryName should be(res0)
When the query can return more than one row, we can use the
list to accumulate the results
in a List.
val countryNames = sql"select name from country order by name" .query[String] .list .transact(xa) .run countryNames.head should be(res0)
This is ok, but there’s not much point reading all the results from the database when we only want the first few rows.
A different approach could be to use the
process that gives us a
scalaz.stream.Process[ConnectionIO, String] which emits the results as they arrive from the
database. By applying a limit with
take we instruct the process to shut everything down
(and clean everything up) after the required number of elements have been emitted. This is
much more efficient than pulling all the rows of the table and then throwing most of them away.
val countryNames = sql"select name from country order by name" .query[String] .process .take(3) .list .transact(xa) .run countryNames.size should be(res0)