十四、SUM(), GROUP BY, HAVING, EXISTS, UCASE(), LCASE()
SUM()
返回数值列的总数
SELECT SUM(column_name)
FROM table;
GROUP BY
结合聚合函数，根据一个或多个列对结果进行分组
SELECT column, function(column_name)
FROM table
WHERE condition
GROUP BY column_name;
HAVING
筛选分组后的各组数据，WHERE子句无法和聚合函数一起使用
SELECT column, function(column1)
FROM table
GROUP BY column1
HAVING condition;
EXISTS
判断是否有记录，可以与NOT一起使用
SELECT column
FROM table
WHERE EXISTS
(SELECT column1 FROM table1 WHERE condition);
UCASE()
把字段值转为大写
SELECT UCASE(column)
FROM table;
LCASE()
SELECT LCASE(column)
FROM table;
