常用函数
AVG() - 返回平均值（SELECT AVG(column_name) FROM table_name WHERE condition;）
COUNT() - 返回行数（SELECT COUNT(1) FROM table_name WHERE condition;）
MAX() - 返回最大值（SELECT MAX(column_name) FROM table_name WHERE condition;）
MIN() - 返回最小值（SELECT MIN(column_name) FROM table_name WHERE condition;）
SUM() - 返回总和（SELECT SUM(column_name) FROM table_name WHERE condition;）
数据表之间的关联
INNER JOIN：如果表中有至少一个匹配，则返回行（取两个表的交集SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name;）
LEFT JOIN：即使右表中没有匹配，也从左表返回所有的行（取左表的全集SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name=table2.column_name; ）
RIGHT JOIN：即使左表中没有匹配，也从右表返回所有的行（取右表的全集SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name = table2.column_name;）
FULL JOIN：只要其中一个表中存在匹配，则返回行（取两个表的并集SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name = table2.column_name;）