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

Question

I am trying to learn the best way to write queries. I also understand the importance of being consistent. Until now, I have randomly used single quotes, double quotes, and backticks without any real thought.

Example:

$query = 'INSERT INTO table (id, col1, col2) VALUES (NULL, val1, val2)';

Also, in the above example, consider that table, col1, val1, etc. may be variables.

What is the standard for this? What do you do?

Graecize

Note that this is a very MySQL specific question. SQL in general (i.e. ISO/ANSI SQL) has a different set of quotes: double quotes are for delimited identifiers, e.g. "tablename", and single quotes are for literals, e.g. 'this is a some text'. Back-ticks are never used in standard SQL. (If you need to include a double quote in an identifier, type it twice as "odd""tablename". Similarly, double single quotes in literals, like 'Conan O''Brien'.) – Transported

Answer 1

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.

Triumphant

"but single quotes are more widely accepted by other RDBMS" - using single quotes for string literals is defined (and required) by the SQL standard – Badillo

@a_horse_with_no_name almost no one use ANSI MySQL ('|' for string concat - really?) – Justin

this isn't true: "MySQL also expects DATE and DATETIME literal values to be single-quoted as strings like '2001-01-01 00:00:00'" – Burch

@Burch Do you have an alternative suggestion? Leaving them unquoted results in them being interpreted as integer arithmetic. It is true that different delimiters than - may be used, but quoting is necessary unless no delimiters are present dev.mysql.com/doc/refman/5.7/en/date-and-time-literals.html – Triumphant

What about column alias and table alias, as in select col1 as c from my_table, for the alias "c", is it better to wrap it with single quote or backtick? – Vivl

@Vivl MySQL docs don't seem to address alias quoting clearly. It will accept single, double, or backtick for aliases but that may be affected by different ANSI SQL modes. I'm not sure what the SQL spec requires for alias quotes -- Personal preference: for consistency I quote them the same as column identifiers - that is, I either backtick them if needed, or leave them unquoted if not. I don't use single or double quotes on aliases. – Triumphant

@MichaelBerkowski You need to quote aliases if they are reserved keywords. The SQL standard is that string literals and dates are wrapped in single quotes. Objects/entities are wrapped in double quotes. MySQL is a weird (albeit pervasive) exception with the use of backticks ``, and SQL Server is kind of a weird (albeit pervasive) exception with the use of square brackets []. In both cases, it is possible to use double quotes and I personally feel like that is a good idea because it enhances portability and readability across different SQL dialects. – Rodolforodolph

But if single quotes are used for string values and the outermost quotes are single quotes, too, the whole line would look like: 'INSERT INTO `$table` (`id`, `col1`, `col2`, `date`) VALUES (NULL, \'' . $val1 . '\', \'' . $val2 . '\', \'' . $date . '\')';. That affects readability. – Hammerless

Does using double quotes for sql query make it vulnerable to php variable injection? Like: "SELECT * FROM $my_table" and the url query is something like ?mytable=$all_tables? – Manolete

@GuneyOzsan Yes, very vulnerable. Never ever use a variable for a table name unless it has been validated against a list of acceptable table names - create an array of permissible names and check the variable matches something in the list to make it safe to use. Otherwise you cannot safely escape a table name variable for use. – Triumphant

Great post. What is the situation with prepared statements? I am getting a weird truncated data error in an established script and am throwing everything at it in the hope something will stick! I am assuming at this point :column is a variable so should NOT be enclosed either in quotes of backticks. Also I am getting the error around a column "ownerType" AFAICS that is not a reserved word ... or is it. ATB – Lancaster

@Lancaster Correct that :column should not be quoted, assuming it is a value. But the name :column suggests you might be trying to use a statement placeholder in place of a column name instead of a value, and that's not allowed. You might need to post a question. – Triumphant

Michael all sorted now thanks. Might I suggest editing the chart to add bound variables. It is absolutely gorgeous and if another poor soul came along checking on :whatever that would be even more lovely. ATB Steve – Lancaster

Answer 2

In MySQL you have:

• 'single quotes' for enclosing string literals
• `Backticks` for enclosing identifiers such as table and column names

And then we have "double quotes" which could be used for one of above purposes at a time depending on MySQL server's sql_mode:

• In stock configuration, the " character can be used to enclose string literals just like '
• In ANSI_QUOTES mode the " character can be used to enclose identifiers just like `

The following query will produce different results (or errors) depending on SQL mode:

SELECT "column" FROM table WHERE foo = "bar"

ANSI_QUOTES disabled

The query will select the string literal "column" where column foo is equal to string "bar"

ANSI_QUOTES enabled

The query will select the column column where column foo is equal to column bar

When to use what

• I suggest that you avoid using " so that your code becomes independent of SQL modes
• Always quote identifiers since it is a good practice

Usufruct

Or alternatively, run your MySQL in ANSI_QUOTES mode to make your SQL skillset more portable, and use ' for strings, " for identifiers and do not use backticks. Adhering to standards is always good – Monopode

For others on how to enable ANSI_QUOTES in a session level: SET SESSION sql_mode = 'ANSI_QUOTES'; or SET SESSION sql_mode = 'ANSI'; . You can then SELECT @@SESSION.sql_mode; to check the value. If you want to restore to the default, the default for MySQL 5.7 is : SET SESSION sql_mode = 'ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION'; – Loosejointed

Answer 3

(There are good answers above regarding the SQL nature of your question, but this may also be relevant if you are new to PHP.)

Perhaps it is important to mention that PHP handles single and double quoted strings differently...

Single-quoted strings are 'literals' and are pretty much WYSIWYG strings. Double-quoted strings are interpreted by PHP for possible variable-substitution (backticks in PHP are not exactly strings; they execute a command in the shell and return the result).

Examples:

$foo = "bar";
echo 'there is a $foo'; // There is a $foo
echo "there is a $foo"; // There is a bar
echo `ls -l`; // ... a directory list

Psych

Answer 4

Backticks are generally used to indicate an identifier and as well be safe from accidentally using the Reserved Keywords.

For example:

Use `database`;

Here the backticks will help the server to understand that the database is in fact the name of the database, not the database identifier.

Same can be done for the table names and field names. This is a very good habit if you wrap your database identifier with backticks.

Check this answer to understand more about backticks.

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Now about Double quotes & Single Quotes (Michael has already mentioned that).

But, to define a value you have to use either single or double quotes. Lets see another example.

INSERT INTO `tablename` (`id, `title`) VALUES ( NULL, title1);

Here I have deliberately forgotten to wrap the title1 with quotes. Now the server will take the title1 as a column name (i.e. an identifier). So, to indicate that it's a value you have to use either double or single quotes.

INSERT INTO `tablename` (`id, `title`) VALUES ( NULL, 'title1');

---

Now, in combination with PHP, double quotes and single quotes make your query writing time much easier. Let's see a modified version of the query in your question.

$query = "INSERT INTO `table` (`id`, `col1`, `col2`) VALUES (NULL, '$val1', '$val2')";

Now, using double quotes in the PHP, you will make the variables $val1, and $val2 to use their values thus creating a perfectly valid query. Like

$val1 = "my value 1";
$val2 = "my value 2";
$query = "INSERT INTO `table` (`id`, `col1`, `col2`) VALUES (NULL, '$val1', '$val2')";

will make

INSERT INTO `table` (`id`, `col1`, `col2`) VALUES (NULL, 'my value 1', 'my value 2')

Scutari

Answer 5

In MySQL, these symbols are used to delimit a query ` ," ,' and () .

1. " or ' are used for enclosing string-like values "26-01-2014 00:00:00" or '26-01-2014 00:00:00' . These symbols are only for strings, not aggregate functions like now, sum, or max.

2. ` is used for enclosing table or column names, e.g. select `column_name` from `table_name` where id='2'

3. ( and ) simply enclose parts of a query e.g. select `column_name` from `table_name` where (id='2' and gender='male') or name='rakesh' .

Sfumato

Answer 6

There has been many helpful answers here, generally culminating into two points.

1. BACKTICKS(`) are used around identifier names.
2. SINGLE QUOTES(') are used around values.

AND as @MichaelBerkowski said

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.

There is a case though where an identifier can neither be a reserved keyword or contain whitespace or characters beyond limited set but necessarily require backticks around them.

EXAMPLE

123E10 is a valid identifier name but also a valid INTEGER literal.

[Without going into detail how you would get such an identifier name], Suppose I want to create a temporary table named 123456e6.

No ERROR on backticks.

DB [XXX]> create temporary table `123456e6` (`id` char (8));
Query OK, 0 rows affected (0.03 sec)

ERROR when not using backticks.

DB [XXX]> create temporary table 123451e6 (`id` char (8));
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '123451e6 (`id` char (8))' at line 1

However, 123451a6 is a perfectly fine identifier name (without back ticks).

DB [XXX]> create temporary table 123451a6 (`id` char (8));
Query OK, 0 rows affected (0.03 sec)

This is completely because 1234156e6 is also an exponential number.

Unskillful

Answer 7

The string literals in MySQL and PHP are the same.

A string is a sequence of bytes or characters, enclosed within either single quote (“'”) or double quote (“"”) characters.

So if your string contains single quotes, then you could use double quotes to quote the string, or if it contains double quotes, then you could use single quotes to quote the string. But if your string contains both single quotes and double quotes, you need to escape the one that used to quote the string.

Mostly, we use single quotes for an SQL string value, so we need to use double quotes for a PHP string.

$query = "INSERT INTO table (id, col1, col2) VALUES (NULL, 'val1', 'val2')";

And you could use a variable in PHP's double-quoted string:

$query = "INSERT INTO table (id, col1, col2) VALUES (NULL, '$val1', '$val2')";

But if $val1 or $val2 contains single quotes, that will make your SQL be wrong. So you need to escape it before it is used in sql; that is what mysql_real_escape_string is for. (Although a prepared statement is better.)

Erasme

Answer 8

In combination of PHP and MySQL, double quotes and single quotes make your query-writing time so much easier.

$query = "INSERT INTO `table` (`id`, `col1`, `col2`) VALUES (NULL, '$val1', '$val2')";

Now, suppose you are using a direct post variable into the MySQL query then, use it this way:

$query = "INSERT INTO `table` (`id`, `name`, `email`) VALUES (' ".$_POST['id']." ', ' ".$_POST['name']." ', ' ".$_POST['email']." ')";

This is the best practice for using PHP variables into MySQL.

Alight

Therefore double quotes are flexible but can't be used as identifiers. – Criminate

Please never ever directly use unescaped user input in your query! – Epizoic

@Epizoic It's just example.I specified that if you are using direct user input then n then........... – Alight

@vipulsorathiya Please specify in your answer that POST variables should be escaped. You're now pointing at using them direct in your query. Bad for beginners who try this... – Gainless

Answer 9

If table cols and values are variables then there are two ways:

With double quotes "" the complete query:

$query = "INSERT INTO $table_name (id, $col1, $col2)
                 VALUES (NULL, '$val1', '$val2')";

Or

 $query = "INSERT INTO ".$table_name." (id, ".$col1.", ".$col2.")
               VALUES (NULL, '".$val1."', '".$val2."')";

With single quotes '':

$query = 'INSERT INTO '.$table_name.' (id, '.$col1.', '.$col2.')
             VALUES (NULL, '.$val1.', '.$val2.')';

Use back ticks `` when a column/value name is similar to a MySQL reserved keyword.

Note: If you are denoting a column name with a table name then use back ticks like this:

`table_name`. `column_name` <-- Note: exclude . from back ticks.

Employment

Answer 10

Single quotes should be used for string values like in the VALUES() list.

Backticks are generally used to indicate an identifier and as well be safe from accidentally using the reserved keywords.

In combination of PHP and MySQL, double quotes and single quotes make your query writing time so much easier.

Leopoldeen

Answer 11

Besides all of the (well-explained) answers, there hasn't been the following mentioned and I visit this Q&A quite often.

In a nutshell; MySQL thinks you want to do math on its own table/column and interprets hyphens such as "e-mail" as e minus mail.

---

Disclaimer: So I thought I would add this as an "FYI" type of answer for those who are completely new to working with databases and who may not understand the technical terms described already.

Whiffle

Answer 12

SQL servers and MySQL, PostgreySQL, Oracle don't understand double quotes("). Thus your query should be free from double quotes(") and should only use single quotes(').

Back-trip(`) is optional to use in SQL and is used for table name, db name and column names.

If you are trying to write query in your back-end to call MySQL then you can use double quote(") or single quotes(') to assign query to a variable like:

let query = "select id, name from accounts";
//Or
let query = 'select id, name from accounts';

If ther's a where statement in your query and/or trying to insert a value and/or an update of value which is string use single quote(') for these values like:

let querySelect = "select id, name from accounts where name = 'John'";
let queryUpdate = "update accounts set name = 'John' where id = 8";
let queryInsert = "insert into accounts(name) values('John')";

//Please not that double quotes are only to be used in assigning string to our variable not in the query
//All these below will generate error

let querySelect = 'select id, name from accounts where name = "John"';
let queryUpdate = 'update accounts set name = "John" where id = 8';
let queryInsert = 'insert into accounts(name) values("John")';

//As MySQL or any SQL doesn't understand double quotes("), these all will generate error.

If you want to stay out of this confusion when to use double quotes(") and single quotes('), would recommend to stick with single quotes(') this will include backslash() like:

let query = 'select is, name from accounts where name = \'John\'';

Problem with double(") or single(') quotes arise when we had to assign some value dynamic and perform some string concatenation like:

let query = "select id, name from accounts where name = " + fName + " " + lName;
//This will generate error as it must be like name = 'John Smith' for SQL
//However our statement made it like name = John Smith

//In order to resolve such errors use
let query = "select id, name from accounts where name = '" + fName + " " + lName + "'";

//Or using backslash(\)
let query = 'select id, name from accounts where name = \'' + fName + ' ' + lName + '\'';

If need further clearance do follow quotes in JavaScript

Breadboard

Answer 13

It is sometimes useful to not use quotes... because this can highlight issues in the code generating the query... For example:

Where x and y are should always be integers...

SELECT * FROM table WHERE x= AND y=0

Is a SQL syntax error... a little lazy but can be useful...

Ridglea