摘要:
---1. 普通的相等连接select e.last_name, d.department_name from employees e, departments dwhere e.department_id = d.department_id;---2 左外连接 LEFT OUTER JOIN select e.last_name,e.department_id,d.department_namefrom employees eleft outer join departments don (e.department_id = d.department_id);select e.last_na
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