Allow ALTER TABLE to operate on columns without COLUMN keyword (#145)

This PR adds support for the following valid ALTER TABLE syntax:
```sql
ALTER TABLE <table-name> ADD <column-name> <data-type>;
```
and
```sql
ALTER TABLE <table-name> DROP <column-name>;
```

Prior to this PR, these queries do not work and result in error
conditions.

For reference, here is the documented SQLite syntax for ALTER TABLE
queries:
https://www.sqlite.org/syntax/alter-table-stmt.html

It shows that the COLUMN keyword is optional, and I've successfully
tested syntax without the COLUMN keyword in both MySQL and SQLite.

Reported by @JanJakes

Author: brandonpayton

tests/WP_SQLite_Translator_Tests.php

@@ -871,7 +871,7 @@ enum_column ENUM('a', 'b', 'c') NOT NULL DEFAULT 'a',
 		);
 	}
 
-	public function testAlterTableAddColumn() {
+	public function testAlterTableAddAndDropColumn() {
 		$result = $this->assertQuery(
 			"CREATE TABLE _tmp_table (
 				name varchar(20) NOT NULL default ''
@@ -905,6 +905,90 @@ public function testAlterTableAddColumn() {
 			),
 			$results
 		);
+
+		$result = $this->assertQuery( 'ALTER TABLE _tmp_table ADD `column2` int;' );
+		$this->assertEquals( '', $this->engine->get_error_message() );
+		$this->assertEquals( 1, $result );
+
+		$this->assertQuery( 'DESCRIBE _tmp_table;' );
+		$results = $this->engine->get_query_results();
+		$this->assertEquals(
+			array(
+				(object) array(
+					'Field'   => 'name',
+					'Type'    => 'varchar(20)',
+					'Null'    => 'NO',
+					'Key'     => '',
+					'Default' => '',
+					'Extra'   => '',
+				),
+				(object) array(
+					'Field'   => 'column',
+					'Type'    => 'int',
+					'Null'    => 'YES',
+					'Key'     => '',
+					'Default' => null,
+					'Extra'   => '',
+				),
+				(object) array(
+					'Field'   => 'column2',
+					'Type'    => 'int',
+					'Null'    => 'YES',
+					'Key'     => '',
+					'Default' => null,
+					'Extra'   => '',
+				),
+			),
+			$results
+		);
+
+		$result = $this->assertQuery( 'ALTER TABLE _tmp_table DROP COLUMN `column`;' );
+		$this->assertEquals( '', $this->engine->get_error_message() );
+		$this->assertEquals( 1, $result );
+
+		$this->assertQuery( 'DESCRIBE _tmp_table;' );
+		$results = $this->engine->get_query_results();
+		$this->assertEquals(
+			array(
+				(object) array(
+					'Field'   => 'name',
+					'Type'    => 'varchar(20)',
+					'Null'    => 'NO',
+					'Key'     => '',
+					'Default' => '',
+					'Extra'   => '',
+				),
+				(object) array(
+					'Field'   => 'column2',
+					'Type'    => 'int',
+					'Null'    => 'YES',
+					'Key'     => '',
+					'Default' => null,
+					'Extra'   => '',
+				),
+			),
+			$results
+		);
+
+		$result = $this->assertQuery( 'ALTER TABLE _tmp_table DROP `column2`;' );
+		$this->assertEquals( '', $this->engine->get_error_message() );
+		$this->assertEquals( 1, $result );
+
+		$this->assertQuery( 'DESCRIBE _tmp_table;' );
+		$results = $this->engine->get_query_results();
+		$this->assertEquals(
+			array(
+				(object) array(
+					'Field'   => 'name',
+					'Type'    => 'varchar(20)',
+					'Null'    => 'NO',
+					'Key'     => '',
+					'Default' => '',
+					'Extra'   => '',
+				),
+			),
+			$results
+		);
 	}
 
 	public function testAlterTableAddNotNullVarcharColumn() {

wp-includes/sqlite/class-wp-sqlite-translator.php

@@ -2921,8 +2921,12 @@ private function execute_alter() {
 			$mysql_index_type = $this->normalize_mysql_index_type( $op_subject );
 			$is_index_op      = (bool) $mysql_index_type;
 
-			if ( 'ADD' === $op_type && 'COLUMN' === $op_subject ) {
-				$column_name = $this->rewriter->consume()->value;
+			if ( 'ADD' === $op_type && ! $is_index_op ) {
+				if ( 'COLUMN' === $op_subject ) {
+					$column_name = $this->rewriter->consume()->value;
+				} else {
+					$column_name = $op_subject;
+				}
 
 				$skip_mysql_data_type_parts = $this->skip_mysql_data_type();
 				$sqlite_data_type           = $skip_mysql_data_type_parts[0];
@@ -2940,7 +2944,7 @@ private function execute_alter() {
 					$column_name,
 					$mysql_data_type
 				);
-			} elseif ( 'DROP' === $op_type && 'COLUMN' === $op_subject ) {
+			} elseif ( 'DROP' === $op_type && ! $is_index_op ) {
 				$this->rewriter->consume_all();
 			} elseif ( 'CHANGE' === $op_type ) {
 				// Parse the new column definition.