Selecting Data

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

How to select data

As we commented in the previous section, the sql string interpolator allows us to create a query to select data from the database.

For instance, sql"select name from country".query[String] defines a Query0[String], which 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 ConnectionIO[List[String]].
  • .vector, which accumulates to a Vector
  • .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 MonadPlus instances.
  • .unique which returns a single value, raising an exception if there is not exactly one row returned.
  • .option which returns an Option, raising an exception if there is more than one row returned.
  • .nel which returns an NonEmptyList, raising an exception if there are no rows returned.
  • See the Scaladoc for Query0 for more information on these and other methods.

Getting info about the countries

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 option method. 

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)