MySQLism: avoid using double quotes for alphanumerical values

According to SQL-99, double quotes should only be used to reference a column name or a table name.

Example:

SELECT "type" FROM access WHERE "type" = ...

This query allows the parser to detect that type is a column and not a function or a reserved word.

Unfortunately, MySQL makes an extensive use of double quotes to represent alphanumerical values. MySQL is very tolerant and allows to write:

SELECT foo FROM bar WHERE foo="this value";

Unfortunately, this code is not portable and will fail on other database systems.

You should always use single quotes for alphanumerical values. The correct syntax is:

SELECT foo FROM bar WHERE foo='this value';

In PHP, the advantage is that you may be able to write:

  $value = "this value";
  $query = "SELECT foo FROM bar WHERE foo='$value';"

So you don't have to escape quote in the PHP code.
This is a small tip to make your code readable!

PostgreSQL does not allow double-quotes in SQL queries. This is because double-quotes are interpreted as escape characters per standard. Instead use only single quotes.

Note also that MySQL accepts backticks for object (table, index, etc.) identifiers. This is non-standard, so PostgreSQL does not support it; instead, use double-quotes if you have to quote a table name (such as having a space in the table name). Better yet, stick to table names which don't require quoting.

Reference: http://drupal.org/node/398252