Contao\Database\Installer::compileCommands PHP Method

    protected function compileCommands()
    {
        $drop = array();
        $create = array();
        $return = array();
        $sql_current = $this->getFromDb();
        $sql_target = $this->getFromDca();
        $sql_legacy = $this->getFromFile();
        // Manually merge the legacy definitions (see #4766)
        if (!empty($sql_legacy)) {
            foreach ($sql_legacy as $table => $categories) {
                foreach ($categories as $category => $fields) {
                    if (is_array($fields)) {
                        foreach ($fields as $name => $sql) {
                            $sql_target[$table][$category][$name] = $sql;
                        }
                    } else {
                        $sql_target[$table][$category] = $fields;
                    }
                }
            }
        }
        // Create tables
        foreach (array_diff(array_keys($sql_target), array_keys($sql_current)) as $table) {
            $return['CREATE'][] = "CREATE TABLE `" . $table . "` (\n  " . implode(",\n  ", $sql_target[$table]['TABLE_FIELDS']) . (!empty($sql_target[$table]['TABLE_CREATE_DEFINITIONS']) ? ',' . "\n  " . implode(",\n  ", $sql_target[$table]['TABLE_CREATE_DEFINITIONS']) : '') . "\n)" . $sql_target[$table]['TABLE_OPTIONS'] . ';';
            $create[] = $table;
        }
        // Add or change fields
        foreach ($sql_target as $k => $v) {
            if (in_array($k, $create)) {
                continue;
            }
            // Fields
            if (is_array($v['TABLE_FIELDS'])) {
                foreach ($v['TABLE_FIELDS'] as $kk => $vv) {
                    if (!isset($sql_current[$k]['TABLE_FIELDS'][$kk])) {
                        $return['ALTER_ADD'][] = 'ALTER TABLE `' . $k . '` ADD ' . $vv . ';';
                    } elseif ($sql_current[$k]['TABLE_FIELDS'][$kk] != $vv && $sql_current[$k]['TABLE_FIELDS'][$kk] != str_replace(' COLLATE ' . \Config::get('dbCollation'), '', $vv)) {
                        $return['ALTER_CHANGE'][] = 'ALTER TABLE `' . $k . '` CHANGE `' . $kk . '` ' . $vv . ';';
                    }
                }
            }
            // Create definitions
            if (is_array($v['TABLE_CREATE_DEFINITIONS'])) {
                foreach ($v['TABLE_CREATE_DEFINITIONS'] as $kk => $vv) {
                    if (!isset($sql_current[$k]['TABLE_CREATE_DEFINITIONS'][$kk])) {
                        $return['ALTER_ADD'][] = 'ALTER TABLE `' . $k . '` ADD ' . $vv . ';';
                    } elseif ($sql_current[$k]['TABLE_CREATE_DEFINITIONS'][$kk] != str_replace('FULLTEXT ', '', $vv)) {
                        $return['ALTER_CHANGE'][] = 'ALTER TABLE `' . $k . '` DROP INDEX `' . $kk . '`, ADD ' . $vv . ';';
                    }
                }
            }
            // Move auto_increment fields to the end of the array
            if (is_array($return['ALTER_ADD'])) {
                foreach (preg_grep('/auto_increment/i', $return['ALTER_ADD']) as $kk => $vv) {
                    unset($return['ALTER_ADD'][$kk]);
                    $return['ALTER_ADD'][$kk] = $vv;
                }
            }
            if (is_array($return['ALTER_CHANGE'])) {
                foreach (preg_grep('/auto_increment/i', $return['ALTER_CHANGE']) as $kk => $vv) {
                    unset($return['ALTER_CHANGE'][$kk]);
                    $return['ALTER_CHANGE'][$kk] = $vv;
                }
            }
        }
        // Drop tables
        foreach (array_diff(array_keys($sql_current), array_keys($sql_target)) as $table) {
            $return['DROP'][] = 'DROP TABLE `' . $table . '`;';
            $drop[] = $table;
        }
        // Drop fields
        foreach ($sql_current as $k => $v) {
            if (!in_array($k, $drop)) {
                // Create definitions
                if (is_array($v['TABLE_CREATE_DEFINITIONS'])) {
                    foreach ($v['TABLE_CREATE_DEFINITIONS'] as $kk => $vv) {
                        if (!isset($sql_target[$k]['TABLE_CREATE_DEFINITIONS'][$kk])) {
                            $return['ALTER_DROP'][] = 'ALTER TABLE `' . $k . '` DROP INDEX `' . $kk . '`;';
                        }
                    }
                }
                // Fields
                if (is_array($v['TABLE_FIELDS'])) {
                    foreach ($v['TABLE_FIELDS'] as $kk => $vv) {
                        if (!isset($sql_target[$k]['TABLE_FIELDS'][$kk])) {
                            $return['ALTER_DROP'][] = 'ALTER TABLE `' . $k . '` DROP `' . $kk . '`;';
                        }
                    }
                }
            }
        }
        // HOOK: allow third-party developers to modify the array (see #3281)
        if (isset($GLOBALS['TL_HOOKS']['sqlCompileCommands']) && is_array($GLOBALS['TL_HOOKS']['sqlCompileCommands'])) {
            foreach ($GLOBALS['TL_HOOKS']['sqlCompileCommands'] as $callback) {
                $this->import($callback[0]);
                $return = $this->{$callback[0]}->{$callback[1]}($return);
            }
        }
        return $return;
    }