[Orcale][Example] DUAL table

DUAL table is is a special one-row (by default value is X), one column (data type VARCHAR2(1)) table which with used for evaluating expressions or calling functions., it also the most simple table designed for fast access

Example 1: SELECT * mean do nothing 

SELECT * FROM dual;

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|    DUMMY    |

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|      X      |

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From the result we can found the default setting of DUAL table: column name: DUMMY, and field value is X.

Example 2: Show string

SELECT 'Hello' FROM dual;

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|   |    'HELLO'   |

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| 1 |    'Hello'   |

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Example 3: Show number

SELECT 4 FROM dual;

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|   |  4  |

-----------

| 1 |  4  |

-----------

Example 4: Show current user

SELECT USER FROM dual;

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|   |    USER    |

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| 1 |  SYSADMIN  |

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Example 5: Show current Oracle date

SELECT SYSDATE FROM dual;

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|   |    SYSDATE    |

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| 1 |   03-8月 -20  |

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Example 6: Show string with build-in function

SELECT UPPER('Hello') FROM dual;

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|   |  UPPER('HELLO') |

-----------------------

| 1 |    'HELLO'      |

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Example 7: Show expression calculation result by accesss the DUAL table:

SELECT 1000+999 FROM dual;

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|   |  1000+999  |

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| 1 |    1999    |

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Reference:

https://en.wikipedia.org/wiki/DUAL_table

https://www.oracletutorial.com/oracle-basics/oracle-dual-table/