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56 lines
1.8 KiB
Markdown
56 lines
1.8 KiB
Markdown
# Introduction
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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
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```SQL
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NULL != NULL -> NULL
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NULL = NULL -> NULL
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```
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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.
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Also, in `where` or `having` conditions, you have to change the expression (not just parameters) if you want to NULL instead of a value:
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```SQL
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a = 7
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a IS NULL
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```
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The library tries to make the usage of NULL reasonably safe in C++:
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# Assigning NULL in INSERT or UPDATE
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In assignments in `insert` or `update` statements, NULL can be represented by `sqlpp::null`.
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```C++
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db(insert_into(t).set(t.gamma = sqlpp::null);
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```
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Note that you cannot assign `sqlpp::null` to columns that are not nullable.
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# Comparing to NULL
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Columns that can be NULL can be compared to NULL using the `is_null` and `is_not_null` member functions.
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```C++
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const auto s = select(all_of(tab)).from(tab).where(tab.alpha == 17 and tab.beta.is_null());
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```
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# Obtaining potential NULL values from a select
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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.
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You can check if a field is NULL by calling the `is_null()` method.
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The value can be accessed by the `.value()` method. It will return a default constructed value in case `is_null() == true`.
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```C++
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for (const auto& row : db(select(all_of(tab)).from(tab).unconditionally()))
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{
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if (not row.alpha.is_null())
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{
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const auto a = row.alpha.value();
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}
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}
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```
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# Future:
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We might switch to `std::optional` to represent nullable columns or values.
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