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Prepared statement

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In database management systems, a prepared statement or parameterized statement is a feature used to execute the same or similar database statements repeatedly with high efficiency. Typically used with SQL statements such as queries or updates, the prepared statement takes the form of a template into which certain constant values are substituted during each execution.

Contents

The typical workflow of using a prepared statement is as follows:

  1. Prepare: The statement template is created by the application and sent to the database management system (DBMS). Certain values are left unspecified, called parameters, placeholders or bind variables (labelled "?" below):
  2. INSERT INTO PRODUCT (name, price) VALUES (?, ?)
  3. The DBMS parses, compiles, and performs query optimization on the statement template, and stores the result without executing it.
  4. Execute: At a later time, the application supplies (or binds) values for the parameters, and the DBMS executes the statement (possibly returning a result). The application may execute the statement as many times as it wants with different values. In this example, it might supply 'Bread' for the first parameter and '1.00' for the second parameter.

As compared to executing SQL statements directly, prepared statements offer two main advantages:

  • The overhead of compiling and optimizing the statement is incurred only once, although the statement is executed multiple times. Not all optimization can be performed at the time the prepared statement is compiled, for two reasons: the best plan may depend on the specific values of the parameters, and the best plan may change as tables and indexes change over time.
  • Prepared statements are resilient against SQL injection, because parameter values, which are transmitted later using a different protocol, need not be correctly escaped. If the original statement template is not derived from external input, SQL injection cannot occur.

    • On the other hand, if a query is executed only once, server-side prepared statements can be slower because of the additional round-trip to the server. Implementation limitations may also lead to performance penalties; for example, some versions of MySQL did not cache results of prepared queries. A stored procedure, which is also precompiled and stored on the server for later execution, has similar advantages. Unlike a stored procedure, a prepared statement is not normally written in a procedural language and cannot use or modify variables or use control flow structures, relying instead on the declarative database query language. Due to their simplicity and client-side emulation, prepared statements are more portable across vendors.

    Software support

    Prepared statements are widely supported by major DBMSs, including MySQL, Oracle, DB2, Microsoft SQL Server, and PostgreSQL. Prepared statements are normally executed through a non-SQL binary protocol, for efficiency and protection from SQL injection, but with some DBMSs such as MySQL are also available using a SQL syntax for debugging purposes.

    A number of programming languages support prepared statements in their standard libraries and will emulate them on the client side even if the underlying DBMS does not support them, including Java's JDBC, Perl's DBI, PHP's PDO and Python's DB-API. Client-side emulation can be faster for queries which are executed only once, by reducing the number of round trips to the server, but is usually slower for queries executed many times. It resists SQL injection attacks equally effectively.

    Many types of SQL injection attacks can be eliminated by disabling literals, effectively requiring the use of prepared statements; as of 2007 only H2 supports this feature.

    Java JDBC

    This example uses Java and the JDBC API:

    Java PreparedStatement provides "setters" (setInt(int), setString(String), setDouble(double), etc.) for all major built-in data types.

    PHP PDO

    This example uses PHP and PHP Data Objects (PDO):

    Alternately:

    Perl DBI

    This example uses Perl and DBI:

    C# ADO.NET

    This example uses C# and ADO.NET:

    ADO.NET SqlCommand will accept any type for the value parameter of AddWithValue, and type conversion occurs automatically. Note the use of "named parameters" (i.e. "@username") rather than "?" - this allows you to use a parameter multiple times and in any arbitrary order within the query command text.

    However, the AddWithValue method should not be used with variable length data types, like varchar and nvarchar. This is because .NET assumes the length of the parameter to be the length of the given value, rather than getting the actual length from the database via reflection. The consequence of this is that a different query plan is compiled and stored for each different length. In general, the maximum number of 'duplicate' plans is the product of the lengths of the variable length columns as specified in the database. For this reason, it is important to use the standard Add method for variable length columns:

    command.Parameters.Add(ParamName, VarChar, ParamLength).Value = ParamValue, where ParamLength is the length as specified in the database.

    Since the standard Add method needs to be used for variable length data types, it is a good habit to use it for all parameter types.

    Python DB-API

    This example uses Python DB-API with SQLite and paramstyle='qmark':

    Magic Direct SQL

    This example uses Direct SQL from Fourth generation language like eDeveloper, uniPaaS and magic XPA from Magic Software Enterprises

    Virtual username Alpha 20 init: 'sister' Virtual password Alpha 20 init: 'yellow' SQL Command: SELECT * FROM users WHERE USERNAME=:1 AND PASSWORD=:2 Input Arguments: 1: username 2: password

    References

    Prepared statement Wikipedia
    (Text) CC BY-SA