When to Use Single Quotes, Double Quotes, and Backticks in MySQL

When to use single quotes, double quotes, and backticks in MySQL

Backticks are to be used for table and column identifiers, but are only necessary when the identifier is a MySQL reserved keyword, or when the identifier contains whitespace characters or characters beyond a limited set (see below) It is often recommended to avoid using reserved keywords as column or table identifiers when possible, avoiding the quoting issue.

Single quotes should be used for string values like in the VALUES() list. Double quotes are supported by MySQL for string values as well, but single quotes are more widely accepted by other RDBMS, so it is a good habit to use single quotes instead of double.

MySQL also expects DATE and DATETIME literal values to be single-quoted as strings like '2001-01-01 00:00:00'. Consult the Date and Time Literals documentation for more details, in particular alternatives to using the hyphen - as a segment delimiter in date strings.

So using your example, I would double-quote the PHP string and use single quotes on the values 'val1', 'val2'. NULL is a MySQL keyword, and a special (non)-value, and is therefore unquoted.

None of these table or column identifiers are reserved words or make use of characters requiring quoting, but I've quoted them anyway with backticks (more on this later...).

Functions native to the RDBMS (for example, NOW() in MySQL) should not be quoted, although their arguments are subject to the same string or identifier quoting rules already mentioned.

Backtick (`)
table & column ───────┬─────┬──┬──┬──┬────┬──┬────┬──┬────┬──┬───────┐
                      ↓     ↓  ↓  ↓  ↓    ↓  ↓    ↓  ↓    ↓  ↓       ↓
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`, `updated`) 
                       VALUES (NULL, 'val1', 'val2', '2001-01-01', NOW())";
                               ↑↑↑↑  ↑    ↑  ↑    ↑  ↑          ↑  ↑↑↑↑↑ 
Unquoted keyword          ─────┴┴┴┘  │    │  │    │  │          │  │││││
Single-quoted (') strings ───────────┴────┴──┴────┘  │          │  │││││
Single-quoted (') DATE    ───────────────────────────┴──────────┘  │││││
Unquoted function         ─────────────────────────────────────────┴┴┴┴┘    

Variable interpolation

The quoting patterns for variables do not change, although if you intend to interpolate the variables directly in a string, it must be double-quoted in PHP. Just make sure that you have properly escaped the variables for use in SQL. (It is recommended to use an API supporting prepared statements instead, as protection against SQL injection).

// Same thing with some variable replacements
// Here, a variable table name $table is backtick-quoted, and variables
// in the VALUES list are single-quoted 
$query = "INSERT INTO `$table` (`id`, `col1`, `col2`, `date`) VALUES (NULL, '$val1', '$val2', '$date')";

Prepared statements

When working with prepared statements, consult the documentation to determine whether or not the statement's placeholders must be quoted. The most popular APIs available in PHP, PDO and MySQLi, expect unquoted placeholders, as do most prepared statement APIs in other languages:

// PDO example with named parameters, unquoted
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`) VALUES (:id, :col1, :col2, :date)";

// MySQLi example with ? parameters, unquoted
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`) VALUES (?, ?, ?, ?)";

Characters requring backtick quoting in identifiers:

According to MySQL documentation, you do not need to quote (backtick) identifiers using the following character set:

ASCII: [0-9,a-z,A-Z$_] (basic Latin letters, digits 0-9, dollar, underscore)

You can use characters beyond that set as table or column identifiers, including whitespace for example, but then you must quote (backtick) them.

Also, although numbers are valid characters for identifiers, identifiers cannot consist solely of numbers. If they do they must be wrapped in backticks.

Where do we use backticks and quotes in SQL?

Backticks (`) are used to indicate database, table, and column names. Unless you're using reserved or conflicting words for table and database names, you'll not need to use them.

Quotes (' or ") are used to delimit strings, and differentiate them from column names.

For example:

SELECT * FROM `database`.`table` WHERE `column` = "value";

As I mentioned, backticks aren't needed, if you use reasonable table and column names:

SELECT * FROM mydb.users WHERE username = "jim";

But strings will always need quotes. This query is comparing the value in the column username against a value in the column bob:

SELECT * FROM mydb.users WHERE username = bob;

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

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.

Use backticks (`) or double quotes (") with Python and SQLite

The SQL standard says that strings must use 'single quotes', and identifiers (such as table and column names), when quoted, must use "double quotes".

For compatibility with MySQL, SQLite also allows to use single quotes for identifiers and double quotes for strings, but only when the context makes the meaning unambiguous. (In SELECT 'rowid' ..., a string is allowed, so a string is what you get.) If possible, always use the standard SQL quotes.

For compatibility with MySQL, SQLite also allows `backticks` for identifiers.

For compatibility with Microsoft databases, SQLite also allows [brackets] for identifiers.

(This works in all SQLite versions.)

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

Double quotes are for names of tables or fields. Sometimes You can omit them. The single quotes are for string constants. This is the SQL standard. In the verbose form, your query looks like this:

select * from "employee" where "employee_name"='elina';

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