What is dynamic SQL?

问题:

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?

回答1:

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)

回答2:

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.

回答3:

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.

回答4:

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)

回答5:

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.

回答6:

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.

回答7:

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.

回答8:

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).

回答9:

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)