摘要:
一、并集 UNION UNION ALL 不会去重,效率更高 SELECT emp_id FROM excellent_emp WHERE year = 2018 UNION SELECT emp_id FROM excellent_emp WHERE year = 2019 ; SELECT em 阅读全文
摘要:
一、相关运算符 IN、ALL、ANY/SOME、EXISTS IN 最好不要有null, SELECT * FROM departments WHERE department_id IN (10, 20, 30) ; SELECT * FROM departments WHERE departmen 阅读全文