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Subqueries. For CodeIgniter.

By Eric Siegel


This is a subquery library for CodeIgniter’s (1.7.x - 2.0.2) active record class. It lets you use the active record methods to create subqueries in your SQL.

It supports SELECT, JOIN, FROM, WHERE, etc. It also supports UNION ALL!

(Yes, you can have subqueries inside subqueries inside UNIONs and UNIONs inside subqueries.)


Put Subquery.php into /application/libraries, then load it in your code using $this->load->library('subquery');. I guess you can add 'subquery' to $autoload['libraries'] (in /application/config/autoload.php), if you want.

CodeIgniter 2.1.x

This library doesn't work with CodeIgniter 2.1.x out of the box. It requires modifications to a file in /system to make it work.

You need to edit /system/database/DB_active_rec.php and modify the signature of _compile_select (should be line 1673). In the older version(s) of CodeIgniter, this function was not protected, so if you remove the protected keyword from the function, my library will work.

(There's probably a reason this function is protected.)

In the develop version of CodeIgniter (which works with this library just fine, by the way), there is a public function that you can use. You can "steal" the get_compiled_select function from the /system/database/DB_query_builder.php file (line 1283).

 * Get SELECT query string
 * Compiles a SELECT query string and returns the sql.
 * @param    string    the table name to select from (optional)
 * @param    bool    TRUE: resets QB values; FALSE: leave QB vaules alone
 * @return    string
public function get_compiled_select($table = '', $reset = TRUE)
    if ($table !== '')

    $select = $this->_compile_select();

    if ($reset === TRUE)

    return $select;

Put this function inside /system/database/DB_active_rec.php.

My library will check for the existance of either a _compile_select or get_compiled_select method. If none of these methods exist, the library will fail to load.


  • start_subquery: Creates a new database object to be used for the subquery
    • Parameters:
      • $statement: SQL statement to put subquery into ('select', 'from', 'join', 'where', 'where_in', etc.)
      • $join_type: JOIN type (only for join statements)
      • $join_on: JOIN ON clause (only for join statements)
    • Returns: CodeIgniter db object to call active record methods on
  • end_subquery: Closes the database object and writes the subquery
    • Parameters:
      • $alias: Alias to use in query, or field to use for WHERE
      • $operator: Operator to use for WHERE ('=', '!=', '<', etc.) / WHERE IN (TRUE for WHERE IN, FALSE for WHERE NOT IN)
        • If it's a SELECT, this parameter will turn it into COALESCE((SELECT ...), $operator) AS $alias
      • $database: Database object to use when dbStack is empty (optional)
    • Returns: Nothing
  • start_union: Creates a new database object to be used for unions
    • Parameters: None
    • Returns: CodeIgniter db object to call active record methods on
  • end_union: Combines all opened db objects into a UNION ALL query
    • Parameters:
      • $database: Database object to use when dbStack is empty (optional)
    • Returns: Nothing


The most basic use of this library is to have a subquery in a SELECT statement. This is very simple. Let's say you want to get this query:

SELECT field1, (SELECT field2 FROM table2 WHERE table1.field3 = table2.field3) as field2X
FROM table1 WHERE field4 = 'test'

You would do this in your code:

$sub = $this->subquery->start_subquery('select');
$sub->where('table1.field3 = table2.field3');
$this->db->where('field4', 'test');

If it's possible that your subquery might return a NULL row, you can set a default value. That's done like this:

$sub = $this->subquery->start_subquery('select');
$sub->where('table1.field3 = table2.field3');
// Note the second parameter here
$this->subquery->end_subquery('field2X', 'field5');
$this->db->where('field4', 'test');

This will generate:

SELECT field1, COALESCE((SELECT field2 FROM table2 WHERE table1.field3 = table2.field3), field5) as field2X
FROM table1 WHERE field4 = 'test'

By passing different values to start_subquery, you can make this library do anyting!

Here's a WHERE IN example:

$this->db->select('field1, field2')->from('table1');
$sub = $this->subquery->start_subquery('where_in');
$sub->select('field3')->from('table2')->where('field2', 'test');
$this->subquery->end_subquery('field4', FALSE);

This will generate:

SELECT field1, field2 FROM table1
WHERE field4 NOT IN (SELECT field3 FROM table2 WHERE field2 = 'test')

UNION queries have a slightly different syntax. For subqueries, every start_subquery needs an end_subquery, but with UNION you only need one end_union - no matter how many start_unions you have.

$sub1 = $this->subquery->start_union();
$sub2 = $this->subquery->start_union();
$sub3 = $this->subquery->start_union();
$this->db->order_by('field1', 'DESC');


An active record subquery library for CodeIgniter. Also contains useful db helper functions.







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