Php Mysql Left join

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$this->staff_data = $wpdb->get_results("SELECT staff.*,department.dep_name FROM " . $this->table_activator->staff_table ." as staff LEFT JOIN ".$this->table_activator->department_table." as department ON staff.department = department.dep_id");

//staff 是 staff_table的别称，department 是department_table别称，选择staff的全部列，department的 dep_name列数据， 当 staff 表里的 department 和 department 表里的dep_id相等的时候

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