如何解决5.5版本mysql下，由json串组成的数组取值问题。

Question

问题详细场景如下：A表中有字段a，a字段类型如[{"PROCESS_ID":"e03e4049-aea2-435e-b5a0-1e808cd5b4ad","PROCESS_VALUE":1.0,"STANDARD_MH":2.00,"IsSelected":true},{"PROCESS_ID":"44742e69-5cb3-4444-a1f9-a833ee945a2d","PROCESS_VALUE":1.0,"STANDARD_MH":1.50,"IsSelected":true},{"PROCESS_ID":"a5c7b02e-2667-4704-9c50-a768f8788a3f","PROCESS_VALUE":1.0,"STANDARD_MH":1.50,"IsSelected":true},{"PROCESS_ID":"7270b3ba-c769-4813-837a-74d550459b86","PROCESS_VALUE":1.0,"STANDARD_MH":3.00,"IsSelected":true},{"PROCESS_ID":"986bac02-0765-40ae-a4fa-173911dffc8b","PROCESS_VALUE":1.0,"STANDARD_MH":1.60,"IsSelected":true},{"PROCESS_ID":"36619e86-5e38-45a5-8167-33bbb6244508","PROCESS_VALUE":1.0,"STANDARD_MH":2.00,"IsSelected":true}]
a字段由若干个json串组成，数量未知。现要计算得到sum(PROCESS_VALUE*STANDARD_MH)，自己捣鼓了很久了，没有解决，求大神帮忙解决下，最好能写个自定义函数，多谢！