Oracle数据库学习笔记_oracle之exist 语句

 oracle之exist 语句

EXISTS里的子查询结果集非空，EXISTS()子句的值就是true。 

EXISTS里的子查询结果集为空，EXISTS()子句的值就是false。 

NOT EXISTS里的子查询结果为非空，NOT EXISTS()字句的值是false

NOT EXISTS里的子查询结果为为空，NOT EXISTS()字句的值是true

例如：

SELECT * FROM S_C_INFORMATION WHERE EXISTS (SELECT 1 FROM DUAL)

能查到结果集

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SELECT * FROM S_C_INFORMATION WHERE EXISTS (SELECT T.NODE_NAME FROM S_C_NODE T WHERE T.NODE_ID='001' )

结果集为空

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SELECT * FROM S_C_NODE_INFORMATION WHERE NOT EXISTS (SELECT T.NODE_NAME FROM S_C_NODE_INFORMATION T WHERE T.NODE_ID='001' )

能查到结果集

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SELECT * FROM S_C_INFORMATION WHERE NOT EXISTS (SELECT 1 FROM DUAL)

结果集为空