本文通過(guò)舉例的方式來(lái)教你如何在Oracle中實(shí)現(xiàn)SELECT TOP N的方法。 1.在Oracle中實(shí)現(xiàn)SELECT TOP N : 由于ORACLE不支持SELECT TOP語(yǔ)句,所以在ORACLE中經(jīng)常是用ORDER BY跟ROWNUM的組合來(lái)實(shí)現(xiàn)SELECT TOP N的查詢。 簡(jiǎn)單地說(shuō),實(shí)現(xiàn)方法如下所示:
SELECT 列名1...列名n FROM (SELECT 列名1...列名n FROM 表名 ORDER BY 列名1...列名n) WHERE ROWNUM <= N(抽出記錄數(shù)) ORDER BY ROWNUM ASC
SELECT 列名1...列名n FROM ( SELECT ROWNUM RECNO, 列名1...列名nFROM (SELECT 列名1...列名n FROM 表名 ORDER BY 列名1...列名n) WHERE ROWNUM <= N(抽出記錄數(shù)) ORDER BY ROWNUM ASC ) WHERE RECNO = M(M <= N)
SELECT ID, NAME FROM ( SELECT ROWNUM RECNO, ID, NAME FROM (SELECT * FROM CUSTOMER ORDER BY NAME) WHERE ROWNUM <= 3 ORDER BY ROWNUM ASC ) WHERE RECNO = 2
SELECT 列名1...列名n FROM ( SELECT ROWNUM RECNO, 列名1...列名nFROM (SELECT 列名1...列名n FROM 表名 ORDER BY 列名1...列名n) WHERE ROWNUM <= N(抽出記錄數(shù)) ORDER BY ROWNUM ASC ) WHERE RECNO = N 那么,2中的例子的SQL語(yǔ)句則為: SELECT ID, NAME FROM ( SELECT ROWNUM RECNO, ID, NAME FROM (SELECT * FROM CUSTOMER ORDER BY NAME) WHERE ROWNUM <= 2 ORDER BY ROWNUM ASC ) WHERE RECNO = 2
結(jié)果為:
ID NAME05 fifth
4.抽出按某種方式排序的記錄集中的第M條記錄開(kāi)始的X條記錄: 上一點(diǎn)所講的僅僅是抽取一條記錄的情況,當(dāng)我們需要抽取多條記錄的時(shí)候,此時(shí)在第二點(diǎn)中的N的取值應(yīng)該是在N >= (M + X - 1)這個(gè)范圍內(nèi),當(dāng)然最經(jīng)濟(jì)的取值就是取等號(hào)的時(shí)候了。當(dāng)然最后的抽取條件也不是RECNO = N了,應(yīng)該是RECNO BETWEEN M AND (M + X - 1)了,所以隨之而來(lái)的SQL語(yǔ)句則為:
SELECT 列名1...列名n FROM ( SELECT ROWNUM RECNO, 列名1...列名nFROM (SELECT 列名1...列名n FROM 表名 ORDER BY 列名1...列名n) WHERE ROWNUM <= N (N >= (M + X - 1)) ORDER BY ROWNUM ASC ) WHERE RECNO BETWEEN M AND (M + X - 1)
SELECT ID, NAME FROM ( SELECT ROWNUM RECNO, ID, NAME FROM (SELECT * FROM CUSTOMER ORDER BY NAME) WHERE ROWNUM <= (2 + 3 - 1) ORDER BY ROWNUM ASC ) WHERE RECNO BETWEEN 2 AND (2 + 3 - 1)