# prepared_statements.rdoc

Path: | doc/prepared_statements.rdoc
Last Update: | Sat Aug 21 15:13:16 -0600 2010

# Prepared Statements and Bound Variables

Sequel has support for prepared statements and bound variables. No matter which database you are using, the Sequel prepared statement/bound variable API remains the same. There is native support for prepared statements/bound variables on the following databases:

- PostgreSQL (using the pg driver, requires type specifiers)
- MySQL (prepared statements only, as the ruby mysql driver doesn‘t support bound variables)
- SQLite (a new native prepared statement is used for each call, though)
- JDBC

Support on other databases is emulated via string interpolation.

## Placeholders

Generally, when using prepared statements (and certainly when using bound variables), you need to put placeholders in your SQL to indicate where you want your bound arguments to appear. Database support and syntax vary significantly for placeholders (e.g. :name, $1, ?). Sequel abstracts all of that and allows you to specify placeholders by using the :$name format for placeholders, e.g.:

      ds = DB[:items].filter(:name=>:$n)

## Bound Variables

Using bound variables for this query is simple:

      ds.call(:select, :n=>'Jim')

This will do the equivalent of selecting records that have the name ‘Jim’. It returns all records, and can take a block that is passed to Dataset#all.

Deleting or returning the first record works similarly:

      ds.call(:first, :n=>'Jim') # First record with name 'Jim'
      ds.call(:delete, :n=>'Jim') # Delete records with name 'Jim'

For inserting/updating records, you should also specify a value hash, which may itself contain placeholders:

      # Insert record with 'Jim', note that the previous filter is ignored
      ds.call(:insert, {:n=>'Jim'}, :name=>:$n)
      # Change name to 'Bob' for all records with name of 'Jim'
      ds.call(:update, {:n=>'Jim', :new_n=>'Bob'}, :name=>$:new_n)

## Prepared Statements

Prepared statement support is similar to bound variable support, but you use Dataset#prepare with a name, and Dataset#call or Database#call later with the values:

      ds = DB[:items].filter(:name=>:$n)
      ps = ds.prepare(:select, :select_by_name)
      ps.call(:n=>'Jim')
      DB.call(:select_by_name, :n=>'Jim') # same as above

The Dataset#prepare method returns a prepared statement, and also stores a copy of the prepared statement in the database for later use. For insert and update queries, the hash to insert/update is passed to prepare:

      ps1 = DB[:items].prepare(:insert, :insert_with_name, :name=>:$n)
      ps1.call(:n=>'Jim')
      DB.call(:insert_with_name, :n=>'Jim') # same as above
      ds = DB[:items].filter(:name=>:$n)
      ps2 = ds.prepare(:update, :update_name, :name=>:$new_n)
      ps2.call(:n=>'Jim', :new_n=>'Bob')
      DB.call(:update_name, :n=>'Jim', :new_n=>'Bob') # same as above

## Database support

### PostgreSQL

If you are using the ruby-postgres or postgres-pr driver, PostgreSQL uses the default emulated support. If you are using ruby-pg, there is native support, but it requires type specifiers most of the time. This is easy if you have direct control over the SQL string, but since Sequel abstracts that, the types have to be specified another way. This is done by adding a __* suffix to the placeholder symbol (e.g. :$name__text, which will be compiled to "$1::text" in the SQL). Prepared statements are always server side.

### SQLite

SQLite supports bound variables and prepared statements exactly the same, since a new native prepared statement is created and executed for each call.

### MySQL

The MySQL ruby driver does not support bound variables, so the the bound variable methods fall back to string interpolation. It uses server side prepared statements.

### JDBC

JDBC supports both prepared statements and bound variables. Whether these are server side or client side depends on the JDBC driver. For PostgreSQL over JDBC, you can add the prepareThreshold=N parameter to the connection string, which will use a server side prepared statement after N calls to the prepared statement.

### All Others

Support is emulated using interpolation.

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