python基礎-面向物件(十五)面向物件進階(三)改變物件的字串顯示__str__,__repr__,__format__
阿新 • • 發佈:2020-12-15
自然連線
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概念
- 自動判斷條件連線,判斷的條件是依據同名欄位
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小結
- 表連線是通過同名欄位來連線的
- 如果沒有同名欄位就返回笛卡爾積
- 同名的連線欄位只顯示一個,並且將該欄位放在最前面
自然內連線(natural join)
MariaDB [sel]> select * from grades natural join resume; +-------+---------+------+----+-----------+ | name | chinese | math | id | skill | +-------+---------+------+----+-----------+ | Sunny | 93 | 96 | 1 | php | | Jerry | 97 | 91 | 3 | php,mysql | +-------+---------+------+----+-----------+ # `2 rows in set (0.023 sec)` MariaDB [sel]> select * from grades; +-------+---------+------+ | name | chinese | math | +-------+---------+------+ | Sunny | 93 | 96 | | Jerry | 97 | 91 | | Marry | 95 | 94 | | Tommy | 98 | 94 | +-------+---------+------+ # `4 rows in set (0.000 sec)` MariaDB [sel]> select * from resume; +----+-------+-----------+ | id | name | skill | +----+-------+-----------+ | 1 | Sunny | php | | 2 | Kimmy | php | | 3 | Jerry | php,mysql | +----+-------+-----------+ # `3 rows in set (0.000 sec)`
自然左外連線(natural left join)
MariaDB [sel]> select * from grades natural left join resume; +-------+---------+------+------+-----------+ | name | chinese | math | id | skill | +-------+---------+------+------+-----------+ | Sunny | 93 | 96 | 1 | php | | Jerry | 97 | 91 | 3 | php,mysql | | Marry | 95 | 94 | NULL | NULL | | Tommy | 98 | 94 | NULL | NULL | +-------+---------+------+------+-----------+ # `4 rows in set (0.001 sec)`
自然右外連線(natural right join)
MariaDB [sel]> select * from grades natural right join resume; +-------+----+-----------+---------+------+ | name | id | skill | chinese | math | +-------+----+-----------+---------+------+ | Sunny | 1 | php | 93 | 96 | | Jerry | 3 | php,mysql | 97 | 91 | | Kimmy | 2 | php | NULL | NULL | +-------+----+-----------+---------+------+ # `3 rows in set (0.000 sec)`