Support sql.Null[T]

[mackee]

Description

This Pull Request introduces support for sql.Null[T] in the go-sqlla library, enhancing type handling capabilities. The key updates include:

• Support for sql.Null[T]: Enables the use of sql.Null[T] in column definitions.
  • Convenient Methods for Null Handling: Generates helpful methods for handling nullable columns using sql.Null[T], distinct from other sql.Null types.
• Internal Refactoring: Includes refactoring of internal static analysis for improved code maintainability and readability.

Additionally, comprehensive tests have been added to ensure the new functionality works correctly.

Example

To define a column using sql.Null[T], you can create the following schema:

type ProductID uint64

//sqlla:table product
type Product struct {
    ID           ProductID          `db:"id,primarykey"`
    SubProductID sql.Null[ProductID] `db:"sub_product_id"`
}

In this schema definition, sub_product_id is recognized as a nullable column, generating corresponding methods. sql.Null[T] allows for both basic and user-defined types.

Unlike other sql.Null* types, you can directly specify ProductID in the Select() WHERE clause without wrapping it in sql.Null[T]:

id := ProductID(42)
// SELECT `id`, `sub_product_id` FROM `product` WHERE sub_product_id = 42;
NewProductSQL().Select().SubProductID(id)

You can also explicitly specify null:

// SELECT `id`, `sub_product_id` FROM `product` WHERE `sub_product_id` IS NULL
NewProductSQL().Select().SubProductIDIsNull()
// SELECT `id`, `sub_product_id` FROM `product` WHERE `sub_product_id` IS NOT NULL
NewProductSQL().Select().SubProductIDIsNotNull()

The IN operator can be specified with multiple ProductID values:

// SELECT `id`, `sub_product_id` FROM `product` WHERE `sub_product_id` IN (1, 2, 3)
NewProductSQL().Select().SubProductIDIn(1, 2, 3)

For Update, you can directly specify ProductID or set null explicitly:

id := ProductID(42)
// UPDATE `product` SET `sub_product_id` = 42
NewProductSQL().Update().SetSubProductID(id)
// UPDATE `product` SET `sub_product_id` = NULL
NewProductSQL().Update().SetSubProductIDToNull()

For Insert(), direct specification or explicit null setting is possible:

id := ProductID(42)
// INSERT INTO `product` (`sub_product_id`) VALUES (42)
NewProductSQL().Insert().ValueSubProductID(id)
// INSERT INTO `product` (`sub_product_id`) VALUES(NULL)
NewProductSQL().Insert().ValueSubProductIDIsNull()

The Insert().OnDuplicateKeyUpdate() method for handling key conflicts also supports these methods:

id := ProductID(42)
// INSERT INTO `product` (`sub_product_id`) VALUES (42) ON DUPLICATE KEY UPDATE SET `sub_product_id` = 42
NewProductSQL().Insert().ValueSubProductID(id).
    OnDuplicateKeyUpdate().
    ValueOnUpdateSubProductID(id)
// INSERT INTO `product` (`sub_product_id`) VALUES (42) ON DUPLICATE KEY UPDATE SET `sub_product_id` = NULL
NewProductSQL().Insert().ValueSubProductID(id).
    OnDuplicateKeyUpdate().
    ValueOnUpdateSubProductIDToNull() 

The Delete() method also allows queries similar to Select():

id := ProductID(42)
// DELETE FROM `product` WHERE sub_product_id = 42
NewProductSQL().Delete().SubProductID(id)
// DELETE FROM `product` WHERE `sub_product_id` IS NULL
NewProductSQL().Delete().SubProductIDIsNull()
// DELETE FROM `product` WHERE `sub_product_id` IS NOT NULL
NewProductSQL().Delete().SubProductIDIsNotNull()
// DELETE FROM `product` WHERE `sub_product_id` IN (1, 2, 3)
NewProductSQL().Delete().SubProductIDIn(1, 2, 3)

These functionalities become available when using sql.Null[T] for column definitions.