sqlpp11/docs/NULL.md

Introduction

Database NULL is a strange beast. It can be compared to anything but that comparison never returns true. It also never returns false, it returns NULL. Even

NULL != NULL    -> NULL
NULL = NULL     -> NULL

A value like that would be pretty unusual in C++. Especially since fields in a result could be either a decent value or NULL. And this can change from result row to result row.

Also, in where or having conditions, you have to change the expression (not just parameters) if you want to NULL instead of a value:

a = 7
a IS NULL

The library tries to make the usage of NULL reasonably safe in C++:

Assigning NULL in INSERT or UPDATE

In assignments in insert or update statements, NULL can be represented by sqlpp::null.

db(insert_into(t).set(t.gamma = sqlpp::null);

Note that you cannot assign sqlpp::null to columns that are not nullable.

Comparing to NULL

Columns that can be NULL can be compared to NULL using the is_null and is_not_null member functions.

const auto s = select(all_of(tab)).from(tab).where(tab.alpha == 17 and tab.beta.is_null());

Obtaining potential NULL values from a select

sqlpp11 can determine whether a result field can be null, based on the columns involved and the structure of your query. If in doubt (for instance due to dynamic parts), it will assume that a field can be NULL.

You can check if a field is NULL by calling the is_null() method.

The value can be accessed by the .value() method. It will return a default constructed value in case is_null() == true.

for (const auto& row :  db(select(all_of(tab)).from(tab).unconditionally()))
{
  if (not row.alpha.is_null())
  {
    const auto a = row.alpha.value();
  }
}

Future:

We might switch to std::optional to represent nullable columns or values.