Safe Haskell	Safe-Inferred
Language	GHC2021

Database.Persist.Sql.Lifted.Expression.Case

Synopsis

case_ :: PersistField a => [(SqlExpr (Value Bool), SqlExpr (Value a))] -> SqlExpr (Value a) -> SqlExpr (Value a)
when_ :: expr (Value Bool) -> () -> expr a -> (expr (Value Bool), expr a)
then_ :: ()
else_ :: expr a -> expr a

Documentation

case_ :: PersistField a => [(SqlExpr (Value Bool), SqlExpr (Value a))] -> SqlExpr (Value a) -> SqlExpr (Value a) #

CASE statement. For example:

select $
return $
case_
   [ when_
       (exists $
       from $ \p -> do
       where_ (p ^. PersonName ==. val "Mike"))
     then_
       (subSelect $
       from $ \v -> do
       let sub =
               from $ \c -> do
               where_ (c ^. PersonName ==. val "Mike")
               return (c ^. PersonFavNum)
       where_ (just (v ^. PersonFavNum) >. subSelect sub)
       return $ count (v ^. PersonName) +. val (1 :: Int)) ]
   (else_ $ val (-1))

This query is a bit complicated, but basically it checks if a person named "Mike" exists, and if that person does, run the subquery to find out how many people have a ranking (by Fav Num) higher than "Mike".

NOTE: There are a few things to be aware about this statement.

This only implements the full CASE statement, it does not implement the "simple" CASE statement.
At least one when_ and then_ is mandatory otherwise it will emit an error.
The else_ is also mandatory, unlike the SQL statement in which if the ELSE is omitted it will return a NULL. You can reproduce this via nothing.

Since: esqueleto-2.1.2

when_ :: expr (Value Bool) -> () -> expr a -> (expr (Value Bool), expr a) #

Syntax sugar for case_.

Since: esqueleto-2.1.2

then_ :: () #

Syntax sugar for case_.

Since: esqueleto-2.1.2

else_ :: expr a -> expr a #

Syntax sugar for case_.

Since: esqueleto-2.1.2