What is the difference between single and double q

问题:

What is the difference between single quotes and double quotes in SQL?

回答1:

Single quotes are used to indicate the beginning and end of a string in SQL. Double quotes generally aren\'t used in SQL, but that can vary from database to database.

Stick to using single quotes.

That\'s the primary use anyway. You can use single quotes for a column alias — where you want the column name you reference in your application code to be something other than what the column is actually called in the database. For example: PRODUCT.id would be more readable as product_id, so you use either of the following:

• SELECT PRODUCT.id AS product_id
• SELECT PRODUCT.id \'product_id\'

Either works in Oracle, SQL Server, MySQL… but I know some have said that the TOAD IDE seems to give some grief when using the single quotes approach.

You do have to use single quotes when the column alias includes a space character, e.g., product id, but it\'s not recommended practice for a column alias to be more than one word.

回答2:

Single quotes delimit a string constant or a date/time constant.

Double quotes delimit identifiers for e.g. table names or column names. This is generally only necessary when your identifier doesn\'t fit the rules for simple identifiers.

See also:

• Do different databases use different name quote?

You can make MySQL use double-quotes per the ANSI standard:

SET GLOBAL SQL_MODE=ANSI_QUOTES

You can make Microsoft SQL Server use double-quotes per the ANSI standard:

SET QUOTED_IDENTIFIER ON

回答3:

Two simple rules for us to remember what to use in which case:

• [S]ingle quotes are for [S]trings ; [D]ouble quotes are for [D]atabase identifiers;
• The ` symbol is the same as the \" symbol. You can use \" with ANSI_QUOTES enabled.

回答4:

In ANSI SQL, double quotes quote object names (e.g. tables) which allows them to contain characters not otherwise permitted, or be the same as reserved words (Avoid this, really).

Single quotes are for strings.

However, MySQL is oblivious to the standard (unless its SQL_MODE is changed) and allows them to be used interchangably for strings.

Moreover, Sybase and Microsoft also use square brackets for identifier quoting.

So it\'s a bit vendor specific.

Other databases such as Postgres and IBM actually adhere to the ansi standard :)

回答5:

I use this mnemonic:

• Single quotes are for strings (one thing)
• Double quotes are for tables names and column names (two things)

This is not 100% correct according to the specs, but this mnemonic helps me (human being).