自定义SQL时，系统内部异常

java.lang.ClassCastException: com.alibaba.druid.sql.ast.statement.SQLUnionQuery cannot be cast to com.alibaba.druid.sql.ast.statement.SQLSelectQueryBlock

com.alibaba.yunbi.biz.customize.sql.SQLStaticChecker.extractQueryBlock(SQLStaticChecker.java:80)
com.alibaba.yunbi.biz.customize.sql.SQLStaticChecker.checkFromTableSource(SQLStaticChecker.java:84)
com.alibaba.yunbi.biz.customize.sql.SQLStaticChecker.checkSyntax(SQLStaticChecker.java:50)

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