**內連接：** 只有進行連接的兩個表中都存在與連接條件相匹配的數據才會被保留下來。 ```sql select * from left_tbl; +--------------+----------------+---------------+--+ | left_tbl.id | left_tbl.name | left_tbl.age | +--------------+----------------+---------------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | | 3 | 王五 | 25 | +--------------+----------------+---------------+--+ select * from right_tbl; +---------------+-----------------+----------------+--+ | right_tbl.id | right_tbl.name | right_tbl.age | +---------------+-----------------+----------------+--+ | 1 | 張三 | 20 | | 2 | 李四 | 22 | | 4 | 王五 | 25 | +---------------+-----------------+----------------+--+ ############ 內連接 ############## 【寫法1】：使用 join/on，join/on是inner join/on 簡寫 select * from left_tbl lt join right_tbl rt on lt.id=rt.id; +--------+----------+---------+--------+----------+---------+--+ | lt.id | lt.name | lt.age | rt.id | rt.name | rt.age | +--------+----------+---------+--------+----------+---------+--+ | 1 | 張三 | 20 | 1 | 張三 | 20 | | 2 | 李四 | 22 | 2 | 李四 | 22 | +--------+----------+---------+--------+----------+---------+--+ 【寫法2】：使用where來代替join/on +--------+----------+---------+--------+----------+---------+--+ | lt.id | lt.name | lt.age | rt.id | rt.name | rt.age | +--------+----------+---------+--------+----------+---------+--+ | 1 | 張三 | 20 | 1 | 張三 | 20 | | 2 | 李四 | 22 | 2 | 李四 | 22 | +--------+----------+---------+--------+----------+---------+--+ ```