Select tables optimized away:这个值意味着仅通过使用索引,优化器可能仅从聚合函数结果中返回一行 No tables used:Query语句中使用from dual 或不含任何from子句 -- explain select now() from dual; 作者:王会举原文链接:sql优化的N种方法_持续更新 发布于 2023-07-05 21:43 赞同4添加评论 分享...
MeaningofFilteringConditions 若从SQL语法来看,where子句中表达的所有过滤条件当然大同小异。但事实并非如此。有些 过滤条件通过关系理论直接作用于select 运算符:where子句检查记录中的字段是否与指定条 件相符。但其实,where 子句中的条件还可以使用另一个关系运算符join。自从SQL92 出现 join 语法后,人们就试图将“joi...
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"Instring" FROM DUAL; Instring --- 14 The next example searches beginning with the third character from the end: SELECT INSTR('CORPORATE FLOOR','OR', -3, 2) "Reversed Instring" FROM DUAL; Reversed Instring --- 2 This example assumes a double-byte database character set. SELECT INSTRB...
SELECT ' MLDN LiXingHua ' , RTRIM(' MLDN LiXingHua ')FROM dual ; TRIM(列 | 字符串) 去掉左右空格 SELECT ' MLDN LiXingHua ' , TRIM(' MLDN LiXingHua ') FROM dual ; 不能去掉中间空格. INSTR(列 | 字符串, 要查找的字符串 , 开始位置 , 出现位置) ...
CREATE OR REPLACE FUNCTION compute_bonus ( emp_id NUMBER, bonus NUMBER ) RETURN NUMBER AUTHID DEFINER IS emp_sal NUMBER; BEGIN SELECT salary INTO emp_sal FROM employees WHERE employee_id = emp_id; RETURN emp_sal + bonus; END compute_bonus; / SELECT compute_bonus(120, 50) FROM DUAL; -...
Additionally, you can detach it from the application window usingDetachfound in the context menu of an already detached tab. After the tab is detached, it's still synchronized with the SQL editor, meaning you can edit and refresh data as long as the SQL editor that produced that tab is op...
SELECT CAST ('1997-10-22' AS DATE) FROM DUAL; SELECT * FROM t1 WHERE CAST (ROWID AS CHAR(5)) = '01234'; 1.9 Oracle Database Lite SQL Datatypes and Literals For a complete list of Oracle Database Lite SQL datatypes, see Appendix C, "Oracle Database Lite Datatypes". For information...
Although not strictly an optimization, an important consideration regarding Always On Availability Group is the fact that, by design, this feature requires the databases to be set in the “Full” recovery model. Meaning, the transaction logs are never discarded until either a full backup is done...
SELECT TO_CHAR(SYSDATE, 'YYYY-MM-DD') AS CURRENT_DATE FROM DUAL; In this example,TO_CHAR()is used to convert the date (SYSDATE) into a character string formatted as ‘YYYY-MM-DD’. The result will be displayed under the column aliasCURRENT_DATE. ...