I would like to use update_all in order to update all records in a database.
Let me assume that the name of the attribute to be update is price.
I succeeded command A.
A
MyModel.update_all(price: "$500")
However, I would like to mysql function(e.g. IF, CONCAT, arithmetic...) in updating.
so I tried command B, but failed and the string value CONCAT('＄', 500) was stored.
B
MyModel.update_all(price: "CONCAT('＄', 500)")
I succeeded when I tried C, but I don't want use it because of the SQL injection risks.
C
MyModel.update_all("price = CONCAT('＄', 500)")
How can I do that without any risk of SQL injection attacks?
Here, I have to use sql function here for some reasons.
Thank you in advance.
You need to create methods corresponding to allowable sql functions that take the params as input. So if you want to provide "CONCAT(..,...)" then user should be able to call a method like this. You can extent it to check type of arguments are allowed and add checks there as well.
def allowed_functions(func_name, all_args_here)
case func_name
when 'concat' then "CONCAT(#{all_args_here})"
end
end
Is it possible to achieve something like this?
Suppose name and plural_name are fields of Animal's table.
Suppose pluralise_animal is a helper function which takes a string and returns its plural literal.
I cannot loop over the animal records for technical reasons.
This is just an example
Animal.update_all("plural_name = ?", pluralise_animal("I WANT THE ANIMAL NAME HERE, the `name` column's value"))
I want something similar to how you can use functions in MySQL while modifying column values. Is this out-of-scope or possible?
UPDATE animals SET plural_name = CONCAT(name, 's') -- just an example to explain what I mean by referencing a column. I'm aware of the problems in this example.
Thanks in advance
I cannot loop over the animal records for technical reasons.
Sorry, this cannot be done with this restriction.
If your pluralizing helper function is implemented in the client, then you have to fetch data values back to the client, pluralize them, and then post them back to the database.
If you want the UPDATE to run against a set of rows without fetching data values back to the client, then you must implement the pluralization logic in an SQL expression, or a stored function or something.
UPDATE statements run in the database engine. They cannot call functions in the client.
Use a ruby script to generate a SQL script that INSERTS the plural values into a temp table
File.open(filename, 'w') do |file|
file.puts "CREATE TEMPORARY TABLE pluralised_animals(id INT, plural varchar(50));"
file.puts "INSERT INTO pluralised_animals(id, plural) VALUES"
Animal.each.do |animal|
file.puts( "( #{animal.id}, #{pluralise_animal(animal.name)}),"
end
end
Note: replace the trailing comma(,) with a semicolon (;)
Then run the generated SQL script in the database to populate the temp table.
Finally run a SQL update statement in the database that joins the temp table to the main table...
UPDATE animals a
INNER JOIN pluralised_animals pa
ON a.id = pa.id
SET a.plural_name = pa.plural;
I just asked an SQL related question, and the first answer was: "This is a situation where dynamic SQL is the way to go."
As I had never heard of dynamic SQL before, I immediately searched this site and the web for what it was. Wikipedia has no article with this title. The first Google results all point to user forums where people ask more or less related questions.
However, I didn't find a clear definition of what a 'dynamic SQL' is. Is it something vendor specific? I work with MySQL and I didn't find a reference in the MySQL handbook (only questions, mostly unanswered, in the MySQL user forums).
On the other hand, I found many references to stored procedures. I have a slightly better grasp of what stored procedures are, although I have never used any. How are the two concepts related? Are they the same thing or does one uses the other?
Basically, what is needed is a simple introduction to dynamic SQL for someone who is new to the concept.
P.S.: If you feel like it, you may have a go at answering my previous question that prompted this one: SQL: How can we make a table1 JOIN table2 ON a table given in a field in table1?
Dynamic SQL is merely where the query has been built on the fly - with some vendors, you can build up the text of the dynamic query within one stored procedure, and then execute the generated SQL. In other cases, the term merely refers to a decision made by code on the client (this is at least vendor neutral)
Other answers have defined what dynamic SQL is, but I didn't see any other answers that attempted to describe why we sometimes need to use it. (My experience is SQL Server, but I think other products are generally similar in this respect.)
Dynamic SQL is useful when you are replacing parts of a query that can't be replaced using other methods.
For example, every time you call a query like:
SELECT OrderID, OrderDate, TotalPrice FROM Orders WHERE CustomerID = ??
you will be passing in a different value for CustomerID. This is the simplest case, and one that can by solved using a parameterized query, or a stored procedure that accepts a parameter, etc.
Generally speaking, dynamic SQL should be avoided in favor of parameterized queries, for performance and security reasons. (Although the performance difference probably varies quite a bit between vendors, and perhaps even between product versions, or even server configuration).
Other queries are possible to do using parameters, but might be simpler as dynamic SQL:
SELECT OrderID, OrderDate, TotalPrice FROM Orders
WHERE CustomerID IN (??,??,??)
If you always had 3 values, this is as easy as the first one. But what if this is a variable-length list? Its possible to do with parameters, but can be very difficult. How about:
SELECT OrderID, OrderDate, TotalPrice FROM Orders WHERE CustomerID = ??
ORDER BY ??
This can't be substituted directly, you can do it with a huge complicated CASE statement in the ORDER BY explicitly listing all possible fields, which may or may not be practical, depending on the number of fields available to sort by.
Finally, some queries simply CAN'T be done using any other method.
Let's say you have a bunch of Orders tables (not saying this is great design), but you might find yourself hoping you can do something like:
SELECT OrderID, OrderDate, TotalPrice FROM ?? WHERE CustomerID = ??
This can't be done using any other methods. In my environment, I frequently encounter queries like:
SELECT (programatically built list of fields)
FROM table1 INNER JOIN table2
(Optional INNER JOIN to table3)
WHERE (condition1)
AND (long list of other optional WHERE clauses)
Again, not saying that this is necessarily great design, but dynamic SQL is pretty much required for these types of queries.
Hope this helps.
Dynamic SQL is simply a SQL statement that is composed on the fly before being executed. For example, the following C# (using a parameterized query):
var command = new SqlCommand("select * from myTable where id = #someId");
command.Parameters.Add(new SqlParameter("#someId", idValue));
Could be re-written using dynamic sql as:
var command = new SqlCommand("select * from myTable where id = " + idValue);
Keep in mind, though, that Dynamic SQL is dangerous since it readily allows for SQL Injection attacks.
Dynamic SQL is a SQL built from strings at runtime. It is useful to dynamically set filters or other stuff.
An example:
declare #sql_clause varchar(1000)
declare #sql varchar(5000)
set #sql_clause = ' and '
set #sql = ' insert into #tmp
select
*
from Table
where propA = 1 '
if #param1 <> ''
begin
set #sql = #sql + #sql_clause + ' prop1 in (' + #param1 + ')'
end
if #param2 <> ''
begin
set #sql = #sql + #sql_clause + ' prop2 in (' + #param2 + ')'
end
exec(#sql)
It is exactly what Rowland mentioned. To elaborate on that a bit, take the following SQL:
Select * from table1 where id = 1
I am not sure which language you are using to connect to the database, but if I were to use C#, an example of a dynamic SQL query would be something like this:
string sqlCmd = "Select * from table1 where id = " + userid;
You want to avoid using dynamic SQL, because it becomes a bit cumbersome to keep integrity of the code if the query get too big. Also, very important, dynamic SQL is susceptible to SQL injection attacks.
A better way of writing the above statement would be to use parameters, if you are using SQL Server.
Rowland is correct, and as an addendum, unless you're properly using parameters (versus just concatonating parameter values inline from provided text, etc.) it can also be a security risk. It's also a bear to debug, etc.
Lastly, whenever you use dynamic SQL unwisely, things are unleashed and children are eaten.
To most databases, every SQL query is "dynamic" meaning that it is a program that is interpreted by the query optimiser given the input SQL string and possibly the parameter bindings ("bind variables").
Static SQL
However, most of the time, that SQL string is not constructed dynamically but statically, either in procedural languages like PL/SQL:
FOR rec IN (SELECT * FROM foo WHERE x = 1) LOOP
-- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ "static SQL"
..
END LOOP;
Or in client / host languages like Java, using JDBC:
try (ResultSet rs = stmt.executeQuery("SELECT * FROM foo WHERE x = 1")) {
// "static SQL" ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
..
}
In both cases, the SQL string is "static" in the language that embeds it. Technically, it will still be "dynamic" to the SQL engine, which doesn't know how the SQL string is constructed, nor that it was a static SQL string.
Dynamic SQL
Sometimes, the SQL string needs to be constructed dynamically, given some input parameters. E.g. the above query might not need any predicate at all in some cases.
You might then choose to proceed to constructing the string dynamically, e.g. in PL/SQL:
DECLARE
TYPE foo_c IS REF CURSOR;
v_foo_c foo_c;
v_foo foo%ROWTYPE;
sql VARCHAR2(1000);
BEGIN
sql := 'SELECT * FROM foo';
IF something THEN
sql := sql || ' WHERE x = 1'; -- Beware of syntax errors and SQL injection!
END IF;
OPEN v_foo_c FOR sql;
LOOP
FETCH v_foo_c INTO v_foo;
EXIT WHEN v_foo_c%NOTFOUND;
END LOOP;
END;
Or in Java / JDBC:
String sql = "SELECT * FROM foo";
if (something)
sql += " WHERE x = 1"; // Beware of syntax errors and SQL injection!
try (ResultSet rs = stmt.executeQuery(sql)) {
..
}
Or in Java using a SQL builder like jOOQ
// No syntax error / SQL injection risk here
Condition condition = something ? FOO.X.eq(1) : DSL.trueCondition();
for (FooRecord foo : DSL.using(configuration)
.selectFrom(FOO)
.where(condition)) {
..
}
Many languages have query builder libraries like the above, which shine most when doing dynamic SQL.
(Disclaimer: I work for the company behind jOOQ)
Is it something vendor specific?
The SQL-92 Standard has a whole chapter on dynamic SQL (chapter 17) but it only applies to FULL SQL-92 and I know of no vendor that has implemented it.
I think what's meant is that you should build the query dynamically before executing it. For your other questions this means that you should select the table name you need first and the use your programming language to build a second query for doing what you want (what you want to do in the other question isn't possible directly like you want).
Seems that it may not be possible, but hey I might as well ask, I could be wrong. Was wondering if there's anyway for perl to update multiple rows using one MySQL call, I'm using DBI.
Any help or feedback would be greatly appreciated, this is possible in MSSQL through ASP and ASP.net so was wondering if also possible through perl on MySQL.
Thank you for your feedback!
First and most important, you absolutely should not interpolate variables directly into your SQL strings. That leaves open the possibility of SQL injection attacks. Even if those variables don't come from user input, it leaves open the possibility of dangerous bugs that can screw up your data.
The MySQL DBD driver does support multiple statements, though it's turned off by default as a safety feature. See mysql_multi_statements under the Class Methods section in the DBD::mysql documentation.
But a much better solution, which solves both problems at once and is more portable, is to use prepared statements and placeholder values.
my $sth = $dbh->prepare("UPDATE LOW_PRIORITY TableName SET E1=?,F1=? WHERE X=?");
Then, get your data in a loop of some sort:
while( $whatever) {
my ( $EC, $MR, $EM ) = get_the_data();
$sth->execute( $EC, $MR, $EM );
}
You only need to prepare the statement once, and the placeholder values are replaced (and guaranteed to be properly quoted) by the DBD driver.
Read more about placeholders in the DBI docs.
You don't need mysql_multi_statements, as friedo suggests.
You need turn off AutoCommit mode before you call the loop containing your UPDATE command:
**$dbh->{AutoCommit} = 0;**
while( $condition ) {
my $myParam = something();
...
$sth->execute( $myParam ); #your prepared UPDATE statement
...
}
**$dbh->commit();**
I've been using the following snippet in developements for years. Now all of a sudden I get a DB Error: no such field warning
$process = "process";
$create = $connection->query
(
"INSERT INTO summery (process) VALUES($process)"
);
if (DB::isError($create)) die($create->getMessage($create));
but it's fine if I use numerics
$process = "12345";
$create = $connection->query
(
"INSERT INTO summery (process) VALUES($process)"
);
if (DB::isError($create)) die($create->getMessage($create));
or write the value directly into the expression
$create = $connection->query
(
"INSERT INTO summery (process) VALUES('process')"
);
if (DB::isError($create)) die($create->getMessage($create));
I'm really confused ... any suggestions?
It's always better to use prepared queries and parameter placeholders. Like this in Perl DBI:
my $process=1234;
my $ins_process = $dbh->prepare("INSERT INTO summary (process) values(?)");
$ins_process->execute($process);
For best performance, prepare all your often-used queries right after opening the database connection. Many database engines will store them on the server during the session, much like small temporary stored procedures.
Its also very good for security. Writing the value into an insert string yourself means that you must write the correct escape code at each SQL statement. Using a prepare and execute style means that only one place (execute) needs to know about escaping, if escaping is even necessary.
Ditto what Zan Lynx said about placeholders. But you may still be wondering why your code failed.
It appears that you forgot a crucial detail from the previous code that worked for you for years: quotes.
This (tested) code works fine:
my $thing = 'abcde';
my $sth = $dbh->prepare("INSERT INTO table1 (id,field1)
VALUES (3,'$thing')");
$sth->execute;
But this next code (lacking the quotation marks in the VALUES field just as your first example does) produces the error you report because VALUES (3,$thing) resolves to VALUES (3,abcde) causing your SQL server to look for a field called abcde and there is no field by that name.
my $thing = 'abcde';
my $sth = $dbh->prepare("INSERT INTO table1 (id,field1)
VALUES (3,$thing)");
$sth->execute;
All of this assumes that your first example is not a direct quote of code that failed as you describe and therefore not what you intended. It resolves to:
"INSERT INTO summery (process) VALUES(process)"
which, as mentioned above causes your SQL server to read the item in the VALUES set as another field name. As given, this actually runs on MySQL without complaint and will fill the field called 'process' with NULL because that's what the field called 'process' contained when MySQL looked there for a value as it created the new record.
I do use this style for quick throw-away hacks involving known, secure data (e.g. a value supplied within the program itself). But for anything involving data that comes from outside the program or that might possibly contain other than [0-9a-zA-Z] it will save you grief to use placeholders.