What makes the big difference between a backtick and an apostrophe?

Question

What is the reason that the following two queries give wildly different results?

MariaDB [mydatabase]> SELECT COUNT(DISTINCT(`price`)) FROM `products`; --         


        

Answer 1

Please find MariaDB's documentation about identifier names and string literals:

https://mariadb.com/kb/en/mariadb/identifier-names/

https://mariadb.com/kb/en/mariadb/string-literals/

Backquotes always quote identifier namens. Single quotes always quote string literals.

Identifier names get replaced by their value, string literals don't:

SELECT `price`, 'price' FROM products;
+-------+-------+
|     1 | price |
|     1 | price |
|     2 | price |
|     3 | price |
|     3 | price |
+-------+-------+

Answer 2

'price' (apostrophes or quotes) is a string. It never changes, so the count is always 1.

`price` (backtics) refers to the column price. So it could be more than 1.

The inner parentheses are irrelevant. COUNT(DISTINCT price) is the same as your backtic version.

• SELECT COUNT(*) FROM tbl WHERE ... is a common way to ask how many rows.
• SELECT foo, COUNT(*) FROM tbl GROUP BY foo is a common way to ask how many rows for each distinct value of foo.
• SELECT foo, COUNT(foo) FROM tbl GROUP BY foo is the same as above, but does not count rows where foo IS NULL.

SELECT DISTINCT ... GROUP BY ... is a nonsense statement. Either use DISTINCT or use GROUP BY.

Answer 3

A straight single quote (') is used for string literals (along with straight double quote (")).

A backtick quote (`) is for quoting identifiers.

Identifiers must be quoted if they match a reserved word, or if they contain special characters. Quoted identifiers also can specify lowercase in case-insensitive fields (which otherwise might be shown as uppercase).

CREATE TABLE MyTable (Field INT);
DESCRIBE MyTable;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| FIELD   | INT         | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

See also ANSI quote mode.