varcols.md

layout	title
article	Working with Unknown Columns

The Scan() function requires you to pass exactly the right number of destination variables. What if you don't know what the query will return?

Scan() 函数要求您准确传递正确数量的目标变量。如果您不知道查询将返回什么，该怎么办？

If you don't know how many columns the query will return, you can use Columns() to find a list of column names. You can examine the length of this list to see how many columns there are, and you can pass a slice into Scan() with the correct number of values. For example, some forks of MySQL return different columns for the SHOW PROCESSLIST command, so you have to be prepared for that or you'll cause an error. Here's one way to do it; there are others:

如果您不知道查询将返回多少列，则可以使用 Columns() 查找列名列表。您可以检查此列表的长度以查看有多少列，并且可以使用正确数量的值将切片传递到 Scan() 。例如，MySQL的一些分支为 SHOW PROCESSLIST 命令返回不同的列，因此您必须为此做好准备，否则您将导致错误。这是一种方法;还有其他人：

cols, err := rows.Columns()
if err != nil {
	// handle the error
} else {
	dest := []interface{}{ // Standard MySQL columns
		new(uint64), // id
		new(string), // host
		new(string), // user
		new(string), // db
		new(string), // command
		new(uint32), // time
		new(string), // state
		new(string), // info
	}
	if len(cols) == 11 {
		// Percona Server
	} else if len(cols) > 8 {
		// Handle this case
	}
	err = rows.Scan(dest...)
	// Work with the values in dest
}

If you don't know the columns or their types, you should use sql.RawBytes.

如果您不知道列或其类型，则应使用sql.RawBytes。

cols, err := rows.Columns() // Remember to check err afterwards
vals := make([]interface{}, len(cols))
for i, _ := range cols {
	vals[i] = new(sql.RawBytes)
}
for rows.Next() {
	err = rows.Scan(vals...)
	// Now you can check each element of vals for nil-ness,
	// and you can use type introspection and type assertions
	// to fetch the column into a typed variable.
}

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