Database.Persist.Sql.Lifted.Expression.Maybe

Documentation

isNothing :: PersistField typ => SqlExpr (Value (Maybe typ)) -> SqlExpr (Value Bool)

IS NULL comparison.

For IS NOT NULL, you can negate this with not_, as in not_ (isNothing (person ^. PersonAge))

Warning: Persistent and Esqueleto have different behavior for != Nothing:

	Haskell	SQL
Persistent	!=. Nothing	IS NOT NULL
Esqueleto	!=. Nothing	!= NULL

In SQL, = NULL and != NULL return NULL instead of true or false. For this reason, you very likely do not want to use !=. Nothing in Esqueleto. You may find these hlint rules helpful to enforce this:

- error: {lhs: v Database.Esqueleto.==. Database.Esqueleto.nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
- error: {lhs: v Database.Esqueleto.==. Database.Esqueleto.val Nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
- error: {lhs: v Database.Esqueleto.!=. Database.Esqueleto.nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}
- error: {lhs: v Database.Esqueleto.!=. Database.Esqueleto.val Nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}

isNothing_ :: PersistField typ => SqlExpr (Value (Maybe typ)) -> SqlExpr (Value Bool)

An alias for isNothing that avoids clashing with the function from Data.Maybe isNothing.

Since: esqueleto-3.5.10.0

just :: SqlExpr (Value typ) -> SqlExpr (Value (Maybe typ))

Analogous to Just, promotes a value of type typ into one of type Maybe typ. It should hold that val . Just === just . val.

nothing :: SqlExpr (Value (Maybe typ))

NULL value.

joinV :: SqlExpr (Value (Maybe (Maybe typ))) -> SqlExpr (Value (Maybe typ))

Join nested Maybes in a Value into one. This is useful when calling aggregate functions on nullable fields.

coalesce :: PersistField a => [SqlExpr (Value (Maybe a))] -> SqlExpr (Value (Maybe a))

COALESCE function. Evaluates the arguments in order and returns the value of the first non-NULL SqlExpression, or NULL (Nothing) otherwise. Some RDBMSs (such as SQLite) require at least two arguments; please refer to the appropriate documentation.

Since: esqueleto-1.4.3

coalesceDefault :: PersistField a => [SqlExpr (Value (Maybe a))] -> SqlExpr (Value a) -> SqlExpr (Value a)

Like coalesce, but takes a non-nullable SqlExpression placed at the end of the SqlExpression list, which guarantees a non-NULL result.

Since: esqueleto-1.4.3