1、笛卡兒積

select * from emp,dep; 

select * from emp,dep where emp.dep_id = dep.id;

select * from emp,dep where emp.dep_id = dep.id and dep.name = "技術(shù)";

2、內(nèi)連接：只取兩張表有對(duì)應(yīng)關(guān)系的記錄

select * from emp inner join dep on emp.dep_id = dep.id;
select * from emp inner join dep on emp.dep_id = dep.id
                                                        where dep.name = "技術(shù)";
        inner join 連接兩個(gè)表, where過(guò)濾條件,只取條件符合的

3、左連接: 在內(nèi)連接的基礎(chǔ)上保留左表沒(méi)有對(duì)應(yīng)關(guān)系的記錄

select * from emp left join dep on emp.dep_id = dep.id;

4、右連接: 在內(nèi)連接的基礎(chǔ)上保留右表沒(méi)有對(duì)應(yīng)關(guān)系的記錄

select * from emp right join dep on emp.dep_id = dep.id;

5、全連接：在內(nèi)連接的基礎(chǔ)上保留左、右面表沒(méi)有對(duì)應(yīng)關(guān)系的的記錄

select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;