diff --git a/system/Database/BaseBuilder.php b/system/Database/BaseBuilder.php
index 3bab4f1569d5..4cc692ebc148 100644
--- a/system/Database/BaseBuilder.php
+++ b/system/Database/BaseBuilder.php
@@ -40,6 +40,7 @@
 
 use CodeIgniter\Database\Exceptions\DatabaseException;
 use CodeIgniter\Database\Exceptions\DataException;
+use Closure;
 
 /**
  * Class BaseBuilder
@@ -701,10 +702,18 @@ protected function whereHaving(string $qb_key, $key, $value = null, string $type
 				}
 				else
 				{
-					$k .= $op;
+					$k .= " $op";
 				}
 
-				$v = " :$bind:";
+				if ($v instanceof Closure)
+				{
+					$builder = $this->cleanClone();
+					$v       = '(' . str_replace("\n", ' ', $v($builder)->getCompiledSelect()) . ')';
+				}
+				else
+				{
+					$v = " :$bind:";
+				}
 			}
 			elseif (! $this->hasOperator($k) && $qb_key !== 'QBHaving')
 			{
@@ -733,13 +742,13 @@ protected function whereHaving(string $qb_key, $key, $value = null, string $type
 	 * Generates a WHERE field IN('item', 'item') SQL query,
 	 * joined with 'AND' if appropriate.
 	 *
-	 * @param string  $key    The field to search
-	 * @param array   $values The values searched on
-	 * @param boolean $escape
+	 * @param string        $key    The field to search
+	 * @param array|Closure $values The values searched on, or anonymous function with subquery
+	 * @param boolean       $escape
 	 *
 	 * @return BaseBuilder
 	 */
-	public function whereIn(string $key = null, array $values = null, bool $escape = null)
+	public function whereIn(string $key = null, $values = null, bool $escape = null)
 	{
 		return $this->_whereIn($key, $values, false, 'AND ', $escape);
 	}
@@ -752,13 +761,13 @@ public function whereIn(string $key = null, array $values = null, bool $escape =
 	 * Generates a WHERE field IN('item', 'item') SQL query,
 	 * joined with 'OR' if appropriate.
 	 *
-	 * @param string  $key    The field to search
-	 * @param array   $values The values searched on
-	 * @param boolean $escape
+	 * @param string        $key    The field to search
+	 * @param array|Closure $values The values searched on, or anonymous function with subquery
+	 * @param boolean       $escape
 	 *
 	 * @return BaseBuilder
 	 */
-	public function orWhereIn(string $key = null, array $values = null, bool $escape = null)
+	public function orWhereIn(string $key = null, $values = null, bool $escape = null)
 	{
 		return $this->_whereIn($key, $values, false, 'OR ', $escape);
 	}
@@ -771,13 +780,13 @@ public function orWhereIn(string $key = null, array $values = null, bool $escape
 	 * Generates a WHERE field NOT IN('item', 'item') SQL query,
 	 * joined with 'AND' if appropriate.
 	 *
-	 * @param string  $key    The field to search
-	 * @param array   $values The values searched on
-	 * @param boolean $escape
+	 * @param string        $key    The field to search
+	 * @param array|Closure $values The values searched on, or anonymous function with subquery
+	 * @param boolean       $escape
 	 *
 	 * @return BaseBuilder
 	 */
-	public function whereNotIn(string $key = null, array $values = null, bool $escape = null)
+	public function whereNotIn(string $key = null, $values = null, bool $escape = null)
 	{
 		return $this->_whereIn($key, $values, true, 'AND ', $escape);
 	}
@@ -790,13 +799,13 @@ public function whereNotIn(string $key = null, array $values = null, bool $escap
 	 * Generates a WHERE field NOT IN('item', 'item') SQL query,
 	 * joined with 'OR' if appropriate.
 	 *
-	 * @param string  $key    The field to search
-	 * @param array   $values The values searched on
-	 * @param boolean $escape
+	 * @param string        $key    The field to search
+	 * @param array|Closure $values The values searched on, or anonymous function with subquery
+	 * @param boolean       $escape
 	 *
 	 * @return BaseBuilder
 	 */
-	public function orWhereNotIn(string $key = null, array $values = null, bool $escape = null)
+	public function orWhereNotIn(string $key = null, $values = null, bool $escape = null)
 	{
 		return $this->_whereIn($key, $values, true, 'OR ', $escape);
 	}
@@ -811,17 +820,17 @@ public function orWhereNotIn(string $key = null, array $values = null, bool $esc
 	 * @used-by whereNotIn()
 	 * @used-by orWhereNotIn()
 	 *
-	 * @param string  $key    The field to search
-	 * @param array   $values The values searched on
-	 * @param boolean $not    If the statement would be IN or NOT IN
-	 * @param string  $type
-	 * @param boolean $escape
+	 * @param string        $key    The field to search
+	 * @param array|Closure $values The values searched on, or anonymous function with subquery
+	 * @param boolean       $not    If the statement would be IN or NOT IN
+	 * @param string        $type
+	 * @param boolean       $escape
 	 *
 	 * @return BaseBuilder
 	 */
-	protected function _whereIn(string $key = null, array $values = null, bool $not = false, string $type = 'AND ', bool $escape = null)
+	protected function _whereIn(string $key = null, $values = null, bool $not = false, string $type = 'AND ', bool $escape = null)
 	{
-		if ($key === null || $values === null)
+		if ($key === null || $values === null || (! is_array($values) && ! ($values instanceof Closure)))
 		{
 			return $this;
 		}
@@ -837,13 +846,20 @@ protected function _whereIn(string $key = null, array $values = null, bool $not
 
 		$not = ($not) ? ' NOT' : '';
 
-		$where_in = array_values($values);
-		$ok       = $this->setBind($ok, $where_in, $escape);
+		if ($values instanceof Closure)
+		{
+			$builder = $this->cleanClone();
+			$ok      = str_replace("\n", ' ', $values($builder)->getCompiledSelect());
+		}
+		else
+		{
+			$ok = $this->setBind($ok, array_values($values), $escape);
+		}
 
 		$prefix = empty($this->QBWhere) ? $this->groupGetType('') : $this->groupGetType($type);
 
 		$where_in = [
-			'condition' => $prefix . $key . $not . " IN :{$ok}:",
+			'condition' => $prefix . $key . $not . ($values instanceof Closure ? " IN ($ok)" : " IN :{$ok}:"),
 			'escape'    => false,
 		];
 
@@ -2640,7 +2656,6 @@ protected function compileWhereHaving(string $qb_key): string
 					{
 						continue;
 					}
-
 					// $matches = array(
 					//	0 => '(test <= foo)',	/* the whole thing */
 					//	1 => '(',		/* optional */
@@ -2968,7 +2983,7 @@ protected function getOperator(string $str, bool $list = false)
 			];
 		}
 
-		return preg_match_all('/' . implode('|', $_operators) . '/i', $str, $match) ? ($list ? $match[0] : $match[0][count($match[0]) - 1]) : false;
+		return preg_match_all('/' . implode('|', $_operators) . '/i', $str, $match) ? ($list ? $match[0] : $match[0][0]) : false;
 	}
 
 	// --------------------------------------------------------------------
@@ -3013,4 +3028,16 @@ protected function setBind(string $key, $value = null, bool $escape = true): str
 	}
 
 	//--------------------------------------------------------------------
+
+	/**
+	 * Returns a clone of a Base Builder with reset query builder values.
+	 *
+	 * @return BaseBuilder
+	 */
+	protected function cleanClone()
+	{
+		return (clone $this)->from([], true)->resetQuery();
+	}
+
+	//--------------------------------------------------------------------
 }
diff --git a/tests/system/Database/Builder/WhereTest.php b/tests/system/Database/Builder/WhereTest.php
index 0e2dbbaa55a1..8e5597df865c 100644
--- a/tests/system/Database/Builder/WhereTest.php
+++ b/tests/system/Database/Builder/WhereTest.php
@@ -1,5 +1,6 @@
 <?php namespace Builder;
 
+use CodeIgniter\Database\BaseBuilder;
 use Tests\Support\Database\MockConnection;
 
 class WhereTest extends \CIUnitTestCase
@@ -118,6 +119,20 @@ public function testWhereCustomString()
 
 	//--------------------------------------------------------------------
 
+	public function testWhereValueClosure()
+	{
+		$builder = $this->db->table('neworder');
+
+		$builder->where('advance_amount <', function (BaseBuilder $builder) {
+			return $builder->select('MAX(advance_amount)', false)->from('orders')->where('id >', 2);
+		});
+		$expectedSQL = 'SELECT * FROM "neworder" WHERE "advance_amount" < (SELECT MAX(advance_amount) FROM "orders" WHERE "id" > 2)';
+
+		$this->assertEquals($expectedSQL, str_replace("\n", ' ', $builder->getCompiledSelect()));
+	}
+
+	//--------------------------------------------------------------------
+
 	public function testOrWhere()
 	{
 		$builder = $this->db->table('jobs');
@@ -191,6 +206,21 @@ public function testWhereIn()
 
 	//--------------------------------------------------------------------
 
+	public function testWhereInClosure()
+	{
+		$builder = $this->db->table('jobs');
+
+		$builder->whereIn('id', function (BaseBuilder $builder) {
+			return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+		});
+
+		$expectedSQL = 'SELECT * FROM "jobs" WHERE "id" IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)';
+
+		$this->assertEquals($expectedSQL, str_replace("\n", ' ', $builder->getCompiledSelect()));
+	}
+
+	//--------------------------------------------------------------------
+
 	public function testWhereNotIn()
 	{
 		$builder = $this->db->table('jobs');
@@ -214,6 +244,21 @@ public function testWhereNotIn()
 
 	//--------------------------------------------------------------------
 
+	public function testWhereNotInClosure()
+	{
+		$builder = $this->db->table('jobs');
+
+		$builder->whereNotIn('id', function (BaseBuilder $builder) {
+			return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+		});
+
+		$expectedSQL = 'SELECT * FROM "jobs" WHERE "id" NOT IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)';
+
+		$this->assertEquals($expectedSQL, str_replace("\n", ' ', $builder->getCompiledSelect()));
+	}
+
+	//--------------------------------------------------------------------
+
 	public function testOrWhereIn()
 	{
 		$builder = $this->db->table('jobs');
@@ -241,6 +286,21 @@ public function testOrWhereIn()
 
 	//--------------------------------------------------------------------
 
+	public function testOrWhereInClosure()
+	{
+		$builder = $this->db->table('jobs');
+
+		$builder->where('deleted_at', null)->orWhereIn('id', function (BaseBuilder $builder) {
+			return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+		});
+
+		$expectedSQL = 'SELECT * FROM "jobs" WHERE "deleted_at" IS NULL OR "id" IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)';
+
+		$this->assertEquals($expectedSQL, str_replace("\n", ' ', $builder->getCompiledSelect()));
+	}
+
+	//--------------------------------------------------------------------
+
 	public function testOrWhereNotIn()
 	{
 		$builder = $this->db->table('jobs');
@@ -267,4 +327,19 @@ public function testOrWhereNotIn()
 	}
 
 	//--------------------------------------------------------------------
+
+	public function testOrWhereNotInClosure()
+	{
+		$builder = $this->db->table('jobs');
+
+		$builder->where('deleted_at', null)->orWhereNotIn('id', function (BaseBuilder $builder) {
+			return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+		});
+
+		$expectedSQL = 'SELECT * FROM "jobs" WHERE "deleted_at" IS NULL OR "id" NOT IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)';
+
+		$this->assertEquals($expectedSQL, str_replace("\n", ' ', $builder->getCompiledSelect()));
+	}
+
+	//--------------------------------------------------------------------
 }
diff --git a/user_guide_src/source/database/query_builder.rst b/user_guide_src/source/database/query_builder.rst
index 613a1e2a2514..977b8e7e7f47 100755
--- a/user_guide_src/source/database/query_builder.rst
+++ b/user_guide_src/source/database/query_builder.rst
@@ -274,12 +274,21 @@ methods:
 		$where = "name='Joe' AND status='boss' OR status='active'";
 		$builder->where($where);
 
-``$builder->where()`` accepts an optional third parameter. If you set it to
-FALSE, CodeIgniter will not try to protect your field or table names.
+    ``$builder->where()`` accepts an optional third parameter. If you set it to
+    FALSE, CodeIgniter will not try to protect your field or table names.
 
-::
+    ::
+
+        $builder->where('MATCH (field) AGAINST ("value")', NULL, FALSE);
+
+#. **Subqueries:**
+    You can use an anonymous function to create a subquery.
+	::
 
-	$builder->where('MATCH (field) AGAINST ("value")', NULL, FALSE);
+        $builder->where('advance_amount <', function(BaseBuilder $builder) {
+            return $builder->select('MAX(advance_amount)', false)->from('orders')->where('id >', 2);
+        });
+        // Produces: WHERE "advance_amount" < (SELECT MAX(advance_amount) FROM "orders" WHERE "id" > 2)
 
 **$builder->orWhere()**
 
@@ -294,44 +303,85 @@ instances are joined by OR::
 Generates a WHERE field IN ('item', 'item') SQL query joined with AND if
 appropriate
 
-::
+    ::
+
+        $names = ['Frank', 'Todd', 'James'];
+        $builder->whereIn('username', $names);
+        // Produces: WHERE username IN ('Frank', 'Todd', 'James')
+
+You can use subqueries instead of an array of values.
+
+    ::
 
-	$names = ['Frank', 'Todd', 'James'];
-	$builder->whereIn('username', $names);
-	// Produces: WHERE username IN ('Frank', 'Todd', 'James')
+        $builder->whereIn('id', function(BaseBuilder $builder) {
+            return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+        });
+
+        // Produces: WHERE "id" IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)
 
 **$builder->orWhereIn()**
 
 Generates a WHERE field IN ('item', 'item') SQL query joined with OR if
 appropriate
 
-::
+    ::
+
+        $names = ['Frank', 'Todd', 'James'];
+        $builder->orWhereIn('username', $names);
+        // Produces: OR username IN ('Frank', 'Todd', 'James')
+
+You can use subqueries instead of an array of values.
 
-	$names = ['Frank', 'Todd', 'James'];
-	$builder->orWhereIn('username', $names);
-	// Produces: OR username IN ('Frank', 'Todd', 'James')
+    ::
+
+        $builder->orWhereIn('id', function(BaseBuilder $builder) {
+            return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+        });
+
+        // Produces: OR "id" IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)
 
 **$builder->whereNotIn()**
 
 Generates a WHERE field NOT IN ('item', 'item') SQL query joined with
 AND if appropriate
 
-::
+    ::
+
+        $names = ['Frank', 'Todd', 'James'];
+        $builder->whereNotIn('username', $names);
+        // Produces: WHERE username NOT IN ('Frank', 'Todd', 'James')
+
+You can use subqueries instead of an array of values.
+
+    ::
+
+        $builder->whereNotIn('id', function(BaseBuilder $builder) {
+            return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+        });
+
+        // Produces: WHERE "id" NOT IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)
 
-	$names = ['Frank', 'Todd', 'James'];
-	$builder->whereNotIn('username', $names);
-	// Produces: WHERE username NOT IN ('Frank', 'Todd', 'James')
 
 **$builder->orWhereNotIn()**
 
 Generates a WHERE field NOT IN ('item', 'item') SQL query joined with OR
 if appropriate
 
-::
+    ::
+
+        $names = ['Frank', 'Todd', 'James'];
+        $builder->orWhereNotIn('username', $names);
+        // Produces: OR username NOT IN ('Frank', 'Todd', 'James')
 
-	$names = ['Frank', 'Todd', 'James'];
-	$builder->orWhereNotIn('username', $names);
-	// Produces: OR username NOT IN ('Frank', 'Todd', 'James')
+You can use subqueries instead of an array of values.
+
+    ::
+
+        $builder->orWhereNotIn('id', function(BaseBuilder $builder) {
+            return $builder->select('job_id')->from('users_jobs')->where('user_id', 3);
+        });
+
+        // Produces: OR "id" NOT IN (SELECT "job_id" FROM "users_jobs" WHERE "user_id" = 3)
 
 ************************
 Looking for Similar Data
@@ -1125,9 +1175,9 @@ Class Reference
 
 	.. php:method:: orWhereIn([$key = NULL[, $values = NULL[, $escape = NULL]]])
 
-		:param	string	$key: The field to search
-		:param	array	$values: The values searched on
-		:param	bool	$escape: Whether to escape values and identifiers
+		:param	string	        $key: The field to search
+		:param	array|Closure   $values: Array of target values, or anonymous function for subquery
+		:param	bool	        $escape: Whether to escape values and identifiers
 		:returns:	BaseBuilder instance
 		:rtype:	object
 
@@ -1136,9 +1186,9 @@ Class Reference
 
 	.. php:method:: orWhereNotIn([$key = NULL[, $values = NULL[, $escape = NULL]]])
 
-		:param	string	$key: The field to search
-		:param	array	$values: The values searched on
-		:param	bool	$escape: Whether to escape values and identifiers
+		:param	string	        $key: The field to search
+		:param	array|Closure   $values: Array of target values, or anonymous function for subquery
+		:param	bool	        $escape: Whether to escape values and identifiers
 		:returns:	BaseBuilder instance
 		:rtype:	object
 
@@ -1147,9 +1197,9 @@ Class Reference
 
 	.. php:method:: whereIn([$key = NULL[, $values = NULL[, $escape = NULL]]])
 
-		:param	string	$key: Name of field to examine
-		:param	array	$values: Array of target values
-		:param	bool	$escape: Whether to escape values and identifiers
+		:param	string	        $key: Name of field to examine
+		:param	array|Closure   $values: Array of target values, or anonymous function for subquery
+		:param	bool            $escape: Whether to escape values and identifiers
 		:returns:	BaseBuilder instance
 		:rtype:	object
 
@@ -1158,9 +1208,9 @@ Class Reference
 
 	.. php:method:: whereNotIn([$key = NULL[, $values = NULL[, $escape = NULL]]])
 
-		:param	string	$key: Name of field to examine
-		:param	array	$values: Array of target values
-		:param	bool	$escape: Whether to escape values and identifiers
+		:param	string	        $key: Name of field to examine
+		:param	array|Closure   $values: Array of target values, or anonymous function for subquery
+		:param	bool	        $escape: Whether to escape values and identifiers
 		:returns:	BaseBuilder instance
 		:rtype:	object