反引号和单引号有什么区别？ 我可以在上面的查询中使用IF语句吗？

Question

In the codeigniter manual writes the following.

$this->db->select() accepts an optional second parameter. If you set it to FALSE, CodeIgniter will not try to protect your field or table names with backticks. This is useful if you need a compound select statement.

$this->db->select('(SELECT SUM(payments.amount) FROM payments 
WHERE payments.invoice_id=4) AS amount_paid', FALSE);
$query = $this->db->get('mytable');
...

And I have the following code from one of CI applications .

$this->db->select('slug, type, 
IF(`value` = "", `default`, `value`) as `value`', FALSE);

Q1. What are the differences between backtick ` and single quote '?

Q2. Can I use IF statement in a query as above?

Q3. What does this mean?

IF(`value` = "", `default`, `value`) as `value`

Answer 1

1. In MySQL, backticks quote names , while single quotes create strings . If you have a column called select , MySQL would throw an syntax error when using this name without backticks -- like in SELECT select FROM foo -- as it would interpret it as keyword which may not occur there.

2. This IF function can be used as a column specification in SELECT statements. See the MySQL reference .

3. This function returns the value from the default column, if value is the empty string. Else it returns the value from value itself. The result will be called value . See the MySQL reference for details.