Selda: A Haskell SQL Library

Chapter 2: Destructive Operations

A database we can't modify in any meaningful way is no good, so let's have a closer look at how to make our database reflect changes to the real world. Throughout this chapter, we're going to work with the table and data presented to the right of the text.

After reading this chapter, you will be able to add, update and delete table rows. You will also gain a better understanding of the types involved, as all examples from this chapter on include full type signatures.

All About Inserts

As we saw in the previous chapter, inserting data into a table is easy. However, the table from the first chapter is often a bit too simplistic. Recall that we used the name of a person as the table's primary key. What if the user changes their name, or if there are multiple persons with the same name?

For reasons like this, we often want the database to keep track of primary keys for us, so for this chapter we've added the pid column to the table, which will at all times contain the unique identifier of each row.

data Person = Person
  { pid  :: ID Person

Since we want the database to manage the identifier for us, we have to add a column attribute, specifying that pid is an auto-incrementing primary key.

people :: Table Person
people = table "people" [#pid :- autoPrimary]

With these simple changes, we can now use the somewhat magical def value when inserting data, to tell the database that we want to use the default value for the pid column.

It is worth noting that def can be used for any column to which we want to assign a default value. However, it is only when used as an auto-incrementing primary key that def ensures the uniqueness of the value; in all other cases it is simply the "least" value of the type, such as False, "" or Nothing.

insertSara :: SeldaM b ()
insertSara = insert_ people [Person def "Sara" 14 Nothing]

If we perform the insert and then select all table rows, we'll find that Sara was added to the table with a unique identifier.

insertThenInspect :: SeldaM b [Person]
insertThenInspect = do
  query (select people)
pid | name      | age | pet
  1 | Velvet    | 19  | Just Dog
  2 | Kobayashi | 23  | Just Dragon
  3 | Miyu      | 10  | Nothing
  4 | Sara      | 14  | Nothing

When using auto-incrementing primary keys, we often want to know what ID a particular row got when we inserted it. The insertWithPK function is handy for this.

insertAndPrintPk :: SeldaM b ()
insertAndPrintPk = do
  pk <- insertWithPK people [Person def Sara 14 Nothing]
  liftIO (print pk)
> withSQLite "database.sqlite" insertAndPrintPk


As time goes by, people age, their pets die, and thus our people table will need to be regulary updated to stay in sync with reality. Fortunately, Selda's got your back:

update :: (MonadSelda m, Relational a)
       => Table a
       -> (Row s a -> Col s Bool)
       -> (Row s a -> Row s a)
       -> m Int

The update function takes a table, a predicate, and an update function as its inputs, and returns the number of rows affected by the update. For each row that matches the given predicate — where the predicate returns true &mdash Selda applies the given update function.

Let's assume, for instance, that we're feeling sorry for anyone who hasn't got a pet, and decide to give every pet-less person a dog:

petsForEveryone :: SeldaM b Int
petsForEveryone = do
  update people
         (\person -> isNull (person ! #pet))
         (\person -> person `with` [#pet := just (literal Dog)])

Here we see the use of with for the first time. The with function takes a table row and a list of updates — expressed as selector-expression pairs — and applies the updates, from left to right, to the table row.

The most basic update is :=, which simply overwrites the column indicated by its given selector with its given expression, but there are also convenient shortcuts for common operations such as incrementing or decrementing values:

ageEveryone :: SeldaM b Int
ageEveryone = do
  update people (const true) (\person -> person `with` [#age += 1])

As expected, this function will increment everyone's age by one.

Conditional Updates

Sometimes it can be useful to update a table row or add it to the table if it doesn't exist. This is sometimes referred to as an upsert. In Selda, this is handled by the upsert function.

upsert :: ( MonadSelda m
          , Relational a
       => Table a
       -> (Row s a -> Col s Bool)
       -> (Row s a -> Row s a)
       -> [a]
       -> m (Maybe (ID a))

This function works much like update, but takes an additional list of elements, to be inserted into the table iff no rows currently in the given table match the given predicate.

To grant a horse to any person named Miyu, and to create a horse owner by that name if none exists, we would do the following:

horseForMiyu :: SeldaM b ()
horseForMiyu = do
  result <- upsert people
                   (\person -> person ! #name .== "Miyu")
                   (\person -> person `with` [#pet := just (literal Horse)])
                   [Person def Miyu 10 Nothing]
  case result of
    Just id -> liftIO $ putStrLn ("person inserted with id " ++ show id)
    Nothing -> liftIO $ putStrLn "update performed; no person inserted"
> withSQLite "database.sqlite" horseForMiyu
update performed; no person inserted

Apart from upsert, there are several less general conditional inserts, which often result in clearer code. Checking out the API documentation is highly recommended.

Deleting Rows

Compared to inserts and updates, there is really not much to say about delete operations.

deleteFrom :: (MonadSelda m, Relational a)
           => Table a
           -> (Row s a -> Col s Bool)
           -> m Int

The delete operation takes a table to delete stuff from, and a predicate determining which rows to delete. Every row that matches the predicate is deleted, and deleteFrom returns the number of deleted rows.

Let's say, for instance, that we want to delete all dragon owners from the table (they've probably gotten eaten already anyway):

deleteDragonOwners :: SeldaM b Int
deleteDragonOwners = do
  deleteFrom persons (\person -> person ! #pet .== just (literal Dragon))
> withSQLite "database.sqlite" deleteDragonOwners
> withSQLite "database.sqlite" (query $ select persons)
pid | name   | age | pet
  1 | Velvet | 19  | Just Dog
  3 | Miyu   | 10  | Nothing