java - 检查 hibernate 中是否存在特定序列

Question

我想运行这样的查询 -

"SELECT nextval('sequenceName')"

但在此之前我想检查 sequenceName 在 Hibernate 中是否存在。

Answer 1

这里的答案给了我一个想法Check if sequence exists in Postgres (plpgsql) , 我不知道这是否是一个完美的解决方案，但您可以使用:

SELECT COUNT(*) 
FROM information_schema.sequences 
WHERE sequence_schema='sch_name' AND sequence_name='sequence_name'

这将为您提供 0(如果不存在)或 1(如果存在)。

所以要解决你的问题，你必须分两次执行你的查询，一个是检查你的序列是否存在，第二个是选择你的序列，所以你的代码应该是这样的:

String sch_name = "sch_test";
String sequence_name = "sequence_name";

//Check if your sequence exist or not
Query q = createNativeQuery("SELECT COUNT(*) FROM        
                             information_schema.sequences 
                             WHERE sequence_schema='" + sch_name + "' 
                             AND sequence_name='" + sequence_name + "'");

int i = (Integer) q.getSingleResult();
int sequence = 0;

//if the sequence exit it will return 1
if(i == 1){
  //increment your sequence and return the result
  q = createNativeQuery("SELECT nextval('" + sequence_name + "')");
  sequence = (Integer) q.getSingleResult();
}else{
   //sequence not exist
}