本文介绍了一种使用SQL查询来统计矩阵中数字出现频率的方法，通过联合多个子查询并对结果进行分组计数，实现了对矩阵a1，a2，a3中所有数字出现次数的有效统计。
查询矩阵a1，a2，a3中每个数字分别出现的次数。
SELECT T.num AS 数字,COUNT(1) AS 出现的次数 FROM ((SELECT a1 AS num FROM bbc_test ORDER BY a1 ASC)
UNION ALL
(SELECT a2 AS num FROM bbc_test ORDER BY a2 ASC)
UNION ALL
(SELECT a3 AS num FROM bbc_test ORDER BY a3 ASC)) T GROUP BY T.num;
结果如下：
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