Function array_to_string_with_null_string 

pub fn array_to_string_with_null_string<Arr: ArrayOrNullableArray + SingleValue, array, del, null>(
    array: array,
    del: del,
    null: null,
) -> array_to_string_with_null_string<Arr, array, del, null>
where
    array: AsExpression<Arr>,
    del: AsExpression<Text>,
    null: AsExpression<Text>,

Available on crate feature postgres_backend only.

Converts each array element to its text representation and concatenates those elements separated by the delimiter string. If null_string is provided and is not NULL, then NULL array entries are represented by that string; otherwise, they are omitted.

Example

// Example with `NULL` representation as a string
let result: String = diesel::select(array_to_string_with_null_string::<
    Array<Nullable<Text>>,
    _,
    _,
    _,
>(
    vec![Some("first"), None::<&str>, Some("third")],
    ",",
    "NULL",
))
.get_result(connection)?;
assert_eq!(result, "first,NULL,third");

// Example without any `NULL` values
let result: String = diesel::select(array_to_string_with_null_string::<
    Array<Nullable<Text>>,
    _,
    _,
    _,
>(vec![Some("first"), Some("second")], ",", "NULL"))
.get_result(connection)?;
assert_eq!(result, "first,second");

// Example with all `NULL` values
let result: String = diesel::select(array_to_string_with_null_string::<
    Array<Nullable<Text>>,
    _,
    _,
    _,
>(vec![None::<&str>, None::<&str>], ",", "NULL"))
.get_result(connection)?;
assert_eq!(result, "NULL,NULL");