Function nullif 

pub fn nullif<'a, V, E1, E2>(
    expr1: E1,
    expr2: E2,
) -> SQLExpr<'a, V, E1::SQLType, Null, <E1::Aggregate as AggregateKind>::Or<E2::Aggregate>, (E1::Sources, E2::Sources)>
where
    V: SQLParam + 'a,
    E1: Expr<'a, V>,
    E2: Expr<'a, V>,
    E1::SQLType: Compatible<E2::SQLType>,
    E2::Aggregate: AggregateKind,

NULL when two values are equal, otherwise the first (NULLIF(a, b)).

Both arguments must have compatible SQL types. The result has the first argument’s type and is always nullable.

Examples

// Treat empty names as missing.
let name = nullif(users.name, "");
assert_eq!(name.sql(), r#"NULLIF ("users"."name", ?)"#);