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111 lines
2.9 KiB
Markdown
111 lines
2.9 KiB
Markdown
# `sqlgen::delete_from`
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The `sqlgen::delete_from` interface provides a type-safe way to delete records from a SQL database. It supports composable query building with `where` clauses to specify which records should be deleted.
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## Usage
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### Basic Delete
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Delete all records from a table:
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```cpp
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const auto conn = sqlgen::sqlite::connect("database.db");
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sqlgen::delete_from<Person>(conn).value();
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```
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This generates the following SQL:
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```sql
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DELETE FROM "Person";
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```
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Note that `conn` is actually a connection wrapped into an `sqlgen::Result<...>`.
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This means you can use monadic error handling and fit this into a single line:
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```cpp
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// sqlgen::Result<Ref<Connection>>
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const auto result = sqlgen::sqlite::connect("database.db").and_then(
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sqlgen::delete_from<Person>);
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```
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Please refer to the documentation on `sqlgen::Result<...>` for more information on error handling.
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### With `where` clause
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Delete specific records using a `where` clause:
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```cpp
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using namespace sqlgen;
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using namespace sqlgen::literals;
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const auto query = delete_from<Person> |
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where("first_name"_c == "Hugo");
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query(conn).value();
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```
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This generates the following SQL:
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```sql
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DELETE FROM "Person"
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WHERE "first_name" = 'Hugo';
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```
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Note that `"..."_c` refers to the name of the column. If such a field does not
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exist on the struct `Person`, the code will fail to compile. `"..."_c` is
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defined in the namespace `sqlgen::literals`.
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You can also use monadic error handling here:
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```cpp
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using namespace sqlgen;
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using namespace sqlgen::literals;
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const auto query = delete_from<Person> |
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where("first_name"_c == "Hugo");
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// sqlgen::Result<Ref<Connection>>
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const auto result = sqlite::connect("database.db").and_then(query);
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```
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## Example: Full Query Composition
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```cpp
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using namespace sqlgen;
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using namespace sqlgen::literals;
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const auto query = delete_from<Person> |
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where("age"_c >= 18 and "last_name"_c == "Simpson");
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const auto result = query(conn);
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if (!result) {
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// Error handling
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}
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```
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This generates the following SQL:
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```sql
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DELETE FROM "Person"
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WHERE ("age" >= 18) AND ("last_name" = 'Simpson');
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```
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It is strongly recommended that you use `using namespace sqlgen` and `using namespace sqlgen::literals`. However,
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if you do not want to do that, you can rewrite the example above as follows:
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```cpp
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const auto query = sqlgen::delete_from<Person> |
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sqlgen::where(sqlgen::col<"age"> >= 18 and sqlgen::col<"last_name"> == "Simpson");
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const auto result = query(conn);
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```
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## Notes
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- The `where` clause is optional - if omitted, all records will be deleted
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- The `Result<Ref<Connection>>` type provides error handling; use `.value()` to extract the result (will throw an exception if there's an error) or handle errors as needed or refer to the documentation on `sqlgen::Result<...>` for other forms of error handling.
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- `"..."_c` refers to the name of the column
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