nulls.md

layout	article
title	Working with NULLs

Nullable columns are annoying and lead to a lot of ugly code. If you can, avoid them. If not, then you'll need to use special types from the database/sql package to handle them, or define your own.

可空列很烦人，导致很多丑陋的代码。如果可以，请避免使用它们。如果没有，那么你需要使用 database/sql 包中的特殊类型来处理它们，或者定义你自己的类型。

There are types for nullable booleans, strings, integers, and floats. Here's how you use them:

有可空的布尔值，字符串，整数和浮点数的类型。这是你如何使用它们：

for rows.Next() {
	var s sql.NullString
	err := rows.Scan(&s)
	// check err
	if s.Valid {
	   // use s.String
	} else {
	   // NULL value
	}
}

Limitations of the nullable types, and reasons to avoid nullable columns in case you need more convincing:

可空类型的限制，以及在需要更有说服力的情况下避免可空列的原因：

1. There's no sql.NullUint64 or sql.NullYourFavoriteType. You'd need to define your own for this.
   没有 sql.NullUint64 或 sql.NullYourFavoriteType。你需要为此定义自己的。
2. Nullability can be tricky, and not future-proof. If you think something won't be null, but you're wrong, your program will crash, perhaps rarely enough that you won't catch errors before you ship them.
   可空性可能很棘手，而且不会出现面向未来的问题。如果您认为某些内容不会为空，但是您错了，那么您的程序将崩溃，或许很少，以至于在发送之前您不会发现错误。
3. One of the nice things about Go is having a useful default zero-value for every variable. This isn't the way nullable things work.
   关于Go的一个好处是为每个变量提供了一个有用的默认零值。这不是可以为空的东西工作的方式。

If you need to define your own types to handle NULLs, you can copy the design of sql.NullString to achieve that.

如果需要定义自己的类型来处理NULL，可以复制sql.NullString的设计来实现它。

If you can't avoid having NULL values in your database, there is another work around that most database systems support, namely COALESCE(). Something like the following might be something that you can use, without introducing a myriad of sql.Null* types.

如果您无法避免在数据库中使用NULL值，那么大多数数据库系统都支持另一种解决方法，即 COALESCE()。像下面这样的东西可能是你可以使用的东西，而不会引入无数的 sql.Null* 类型。

rows, err := db.Query(`
	SELECT
		name,
		COALESCE(other_field, '') as otherField
	WHERE id = ?
`, 42)

for rows.Next() {
	err := rows.Scan(&name, &otherField)
	// ..
	// If `other_field` was NULL, `otherField` is now an empty string. This works with other data types as well.
}

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