使用Group By 语句进行聚合操作时输出的方式都是以行的方式呈现,例如:
SELECT T.MESSAGE_TYPE AS MESSAGE_TYPE, COUNT(1) AS COUNT FROM TABLE_A T WHERE T.CREATED_AT = '2019-03-22 10:20:20' GROUP BY T.MESSAGE_TYPE;

但是通常我们更希望以列的方式展现在表格中,这样更为直观,那么可以改写成如下SQL:
SELECT COUNT(DECODE(T.MESSAGE_TYPE, 'AT+CMD', 1, NULL)) AS AT_CMD, COUNT(DECODE(T.MESSAGE_TYPE, 'AT+CFG', 1, NULL)) AS AT_CFG, COUNT(DECODE(T.MESSAGE_TYPE, 'AT+QRY', 1, NULL)) AS AT_QRY FROM TABLE_A T WHERE T.CREATED_AT = '2019-03-22 10:20:20';

列的方式聚合同一字段不同的值