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在Oracle数据库中,LEVEL函数是一个伪列(pseudocolumn),它用于查询时生成一个连续的整数序列
SELECT employee_id, first_name, last_name, manager_id, LEVEL
FROM employees
START WITH manager_id IS NULL
CONNECT BY PRIOR employee_id = manager_id;
SELECT employee_id, first_name, last_name, manager_id, LEVEL
FROM employees
START WITH manager_id IS NULL
CONNECT BY PRIOR employee_id = manager_id
AND LEVEL <= 3;
SELECT LEVEL AS sequence
FROM DUAL
CONNECT BY LEVEL <= 10;
SELECT *
FROM (
SELECT t.*, ROWNUM AS rn
FROM (
SELECT *
FROM your_table
ORDER BY some_column
) t
WHERE ROWNUM <= 20
)
WHERE rn >= 11;
SELECT SYSDATE + LEVEL - 1 AS date_sequence
FROM DUAL
CONNECT BY LEVEL <= 7;
总之,LEVEL函数在Oracle中具有多种高级应用,可以帮助你更有效地处理各种数据查询和操作需求。
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