SQL查询和优化（二）

一、模糊查询

CREATE OR REPLACE VIEW v AS
SELECT 'ABCEDF' AS vname FROM dual
UNION ALL
CREATE OR REPLACE VIEW v AS
SELECT '_BCEFG' AS vname FROM dual
UNION ALL
CREATE OR REPLACE VIEW v AS
SELECT '_BCEDF' AS vname FROM dual
UNION ALL
CREATE OR REPLACE VIEW v AS
SELECT '_\BCEDF' AS vname FROM dual
UNION ALL
CREATE OR REPLACE VIEW v AS
SELECT 'XYCEG' AS vname FROM dual ;

要求一、查出vname中包含字符串“CED”的

SQL> SELECT * FROM v WHERE vname LIKE '%CED%';

要求二、查出vname中包含字符串“_BCE”的

SQL> SELECT * FROM v WHERE vname LIKE '\_BCE%' ESCAPE '\';

要求三、查出vname中包含字符串“\BCE”的

SQL> SELECT * FROM v WHERE vname LIKE '\\BCE%' ESCAPE '\';

二、 限制返回的行数

SQL> SELECT * FROM emp WHERE rownum <= 2;

如果直接用rownum=2来查询，会出现下面的情况

SQL> SELECT * FROM emp WHERE rownum = 2;
no rows selected

只取第二行数据应该这样

SELECT * 
 FROM(SELECT rownum AS sn, emp.* FROM emp WHERE rownum <= 2)
WHERE sn=2;

三、 从表中随机返回N条数据

SELECT empno, ename
 FROM (SELECT empno,ename FROM emp ORDER BY dbms_random.value())
 WHERE rownum<=3;

这里要嵌套一层，先随机排序，再取数据。
错误的写法如下：先取数据，再随机排序

SELECT empno, ename FROM emp WHERE rownum<=3 ORDER BY dbms_random.value();


选自《Oracle 查询优化改写技巧与案例》
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