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?
I've been reading answers to similar questions on here for about 20 minutes, but it seems like there is no definitive answer to this question.
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.
There are two types of quotes in MySQL:
' for enclosing string literals
` for enclosing identifiers such as table and column names
And then there is " which is a special case. It could be used for one of above-mentioned purposes at a time depending on MySQL server's sql_mode:
By default 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 (quite a few questions on SO discuss this)
(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
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.
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')
In MySQL, these symbols are used to delimit a query ` ," ,' and () .
" 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.
` is used for enclosing table or column names, e.g. select `column_name` from `table_name` where id='2'
( and ) simply enclose parts of a query e.g. select `column_name` from `table_name` where (id='2' and gender='male') or name='rakesh' .
There has been many helpful answers here, generally culminating into two points.
BACKTICKS(`) are used around identifier names.
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.
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.)
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.
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.
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.
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.
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
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...
It seems PostgreSQL does not allow to create a database table named 'user'. But MySQL will allow to create such a table.
Is that because it is a key word? But Hibernate cannot identify any issue (even if we set the PostgreSQLDialect).
user is a reserved word and it's usually not a good idea use reserved words for identifiers (tables, columns).
If you insist on doing that you have to put the table name in double quotes:
create table "user" (...);
But then you always need to use double quotes when referencing the table. Additionally the table name is then case-sensitive. "user" is a different table name than "User".
If you want to save yourself a lot of trouble use a different name. users, user_account, ...
More details on quoted identifiers can be found in the manual: http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
It is possible to specify tablename with JPA with next syntax:
#Table(name="\"user\"")
We had this same issue time ago, and we just changed the table name from user to app_user. Due to the use of Hibernate/JPA. We thought it would be easier this way.
Hope this little fix will help someone else.
You can create a table user in a schema other than public.
The example:
CREATE SCHEMA my_schema;
CREATE TABLE my_schema.user(...);
Trailing underscore
The SQL standard explicitly promises to never use a trailing underscore in any keyword or reserved word.
So, to avoid conflicts with any of the over a thousand keywords and reserved words used by various database engines, I name all my database identifiers with a trailing underscore. (Yes, really, over a thousand keywords reserved — I counted them.)
Change this:
CREATE TABLE user ( … ) ;
… to this:
CREATE TABLE user_ ( … ) ;
I do this as a habit for all database names: schemas, tables, columns, indexes, etc.
As an extra benefit, this practice makes quite clear in documentation, email, and such when referring to a programming language variable named user versus the database column user_. Anything with a trailing underscore is obviously from the database side.
I am trying to convert from MS Access to DerbyDB. However some varchar fields have 'special' characters, such as newlines, tabs, percent marks, foreign characters etc.
I created a quick method...
public String charCheck(String s)
{
errLog.add(1, "converting string from " + s);
s.replaceAll("'", "''");//an apostrophe is escaped with an apostrophy in Derby...
s.replaceAll("%", "\\%");//a percent sign
s.replaceAll("\\s+n", " ");//whitespace characters (newlines and tabs etc)
s.replaceAll("/", "\\/");//the 'divide' \ character,
s.replaceAll("<", "\\<");//mathematical symbol less than
s.replaceAll(">", "\\>");//mathematical symbol greater than
errLog.add(1, "to " + s);
return s;
}//end method
Which I run whenever I determine that I need a varchar (or long varchar) data type. The strange thing is that my error log prints out the messages, but in the output whitespace characters do not appear to change (ie tabs and new lines, do not get converted to a simple space) and any apostrophe in the string is not replaced.
a sample of the output of this method produces the following.
converting string from 2. FIN DE L’ESSAI
to 2. FIN DE L’ESSAI
So the string remains obviously unchanged, which upsets derbyDB when I run the insert statement, also I am not finding any obvious documentation on the escape sequence for inserting multiple records into a table, I would like to use a statement then add the escape keyword after it, ie
stmt.execute("{call "+ sqlInsertStatement +"}{escape '" + escapeCharacter +"'" );
I also read from the docs that the escape keyword may not be usefull in the above statement, if so how can I te
I need to know where to go to sort the insert error that I get.
If I copy and past the insert statement directly into ij, then remove the special characters the record will insert fine, I just don't understand why it isn't being converted in the first instance.
I have also tried surrounding varchar and longvarchar fields with double quotes, but again derby kicks out an error saying that a double quote was found!
I want to get this sorted as I feel like I am so close...
Thanks in advance
Strings are immutable, all operations you perform on them results new Strings. You need to assign current reference to new String.
Example:
s= s.replaceAll("'", "''");
If it is just replace, then replace() may be best option than using replaceAll()
Instead of working through the String, turn the String into a char array, then use a do...while loop to work through each char in turn using if tests to replace each char. Then turn the array back to a String then return it
I am trying to query phone number stored in the contacts database of android.
I am providing an input sting for comparison and I need the query to pick only those rows which matches this input string.
While querying the contacts db, I would like the query to ignore all the special characters contained in a number and compare the input string with only the digits contained in the field.
eg; if the stored number is (944) 6-129-337 , the query should compare the input string with 9446129337 and not with (944) 6-129-337 .
Following is the code with which I query the db and fetch the numbers.
contactCursor = getContentResolver().query(uri, projection,
ContactsContract.Data.MIMETYPE + "='" + ContactsContract.CommonDataKinds.Phone.CONTENT_ITEM_TYPE + "' AND " +
ContactsContract.CommonDataKinds.Phone.NUMBER + " LIKE ? ",new String[] { input_string }, null);
The problem I have here is, it just does a plain search in each phone number field without internally removing(using regexp or modifying the query?) the special characters.
I do not know how to do that cleaning up of special characters inside query. Any help in this regard which would help me resolve this query issue will be well appreciated
Thanks,
Anees
As a hack that may work for you... you could pass your number using wildcards between every digit, and then re-filter the results in your Java code... e.g. ... LIKE '%9%4%4%6%1%2%9%3%3%7%' ... is liable to return very few results, including (944) 6-129-337 and (perhaps) +1 (944) 612-9337; but you can collect these results, strip the non-digit values in your own code, and compare them. If you have very short search strings, though, your number of "false positive" hits will be much greater. Unfortunately, I haven't tested this with the Android content system, so it's plausible that you could overflow the number of wildcard characters permitted, or similar :-(
Use This
Uri uri = Uri.parse("content://com.android.contacts/data/phones/filter/%915894%");
Return Only your need.