(1)優化前：使用or的時候，SQL執行時間1.47smysql>?select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?e.emp_no=32000?or?d.from_date='1996-11-24';

58?rows?in?set?(1.47?sec)

mysql>?desc?select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?e.emp_no=32000?or?d.from_date='1996-11-24';

+----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+

|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys??|?key?????|?key_len?|?ref????????????????|?rows???|?filtered?|?Extra???????|

+----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+

|??1?|?SIMPLE??????|?e?????|?NULL???????|?ALL??|?PRIMARY????????|?NULL????|?NULL????|?NULL???????????????|?299335?|???100.00?|?NULL????????|

|??1?|?SIMPLE??????|?d?????|?NULL???????|?ref??|?PRIMARY,emp_no?|?PRIMARY?|?4???????|?employees.e.emp_no?|??????1?|???100.00?|?Using?where?|

+----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+

2?rows?in?set,?1?warning?(0.00?sec)

(2)優化后：相同結果集，使用union的時候，SQL執行時間只需0.23smysql>?select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?e.emp_no=32000

->??union

->?select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?d.from_date='1996-11-24';

58?rows?in?set?(0.23?sec)

mysql>?desc?select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?e.emp_no=32000?union??select?e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date?from?employees?e?left?join?dept_emp?d?on?e.emp_no=d.emp_no?where?d.from_date='1996-11-24';

+----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+

|?id?|?select_type??|?table??????|?partitions?|?type???|?possible_keys??|?key?????|?key_len?|?ref????????????????|?rows???|?filtered?|?Extra???????????|

+----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+

|??1?|?PRIMARY??????|?e??????????|?NULL???????|?const??|?PRIMARY????????|?PRIMARY?|?4???????|?const??????????????|??????1?|???100.00?|?NULL????????????|

|??1?|?PRIMARY??????|?d??????????|?NULL???????|?ref????|?PRIMARY,emp_no?|?PRIMARY?|?4???????|?const??????????????|??????1?|???100.00?|?NULL????????????|

|??2?|?UNION????????|?d??????????|?NULL???????|?ALL????|?PRIMARY,emp_no?|?NULL????|?NULL????|?NULL???????????????|?331143?|????10.00?|?Using?where?????|

|??2?|?UNION????????|?e??????????|?NULL???????|?eq_ref?|?PRIMARY????????|?PRIMARY?|?4???????|?employees.d.emp_no?|??????1?|???100.00?|?NULL????????????|

|?NULL?|?UNION?RESULT?|??|?NULL???????|?ALL????|?NULL???????????|?NULL????|?NULL????|?NULL???????????????|???NULL?|?????NULL?|?Using?temporary?|

+----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+

5?rows?in?set,?1?warning?(0.00?sec)

總結

以上是生活随笔為你收集整理的mysql union left join_Case：MySQL使用left join的时候or改写成union可以提高效率的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。