SELECT 1 FROM TABLE_NAME means, "Return 1 from the table". It is pretty unremarkable on its own, so normally it will be used with WHERE and often EXISTS (as @gbn notes, this is not necessarily best practice, it is, however, common enough to be noted, even if it isn't really meaningful (that said, I will use it because others use it and it is "more obvious" immediately.

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I find that examples are the best way for me to learn about code, even with the explanation above. Example 1 Queries like 'select 1 + 1' can be run without a "from" clause/table name. MySQL allows DUAL to be specified as a table in queries that do not need data from any tables. It is suitable for use in selecting a result function such as SYSDATE() or USER(), although it is not essential.

select 1 from dual; RAW Paste Data . Public Pastes. My Log File. HTML 5 | 16 min ago . My Log File. HTML 5 | 16 min ago .

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Example. SELECT * FROM Customers;. Try it Yourself ». Click on the "Try by completing the SQL course. Get certified w 3 s c h o o l s C E R T I F I E D . 2 0 2 1 .

The SQL statement SELECT ROUND(45.926, -1) FROM DUAL; a) is illegal b) prints garbage c) prints 045.926 d) prints 50.

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One further question that this brings up for me is whether or not I actually need to have a validation query, or if I can maybe get a performance boost by disabling it or at least reducing the frequency with which The validationQuery was set to “select 1 from DUAL” (obviously) and apparently the connection pool defaults testOnBorrow and testOnReturn to true when a validation query is non-null.
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MariaDB ignores the clauses. MariaDB does not require FROM DUAL if no tables are referenced. FROM DUAL could be used when you only SELECT computed values, but require a WHERE clause, perhaps to test that a script correctly handles empty resultsets: SELECT 1 FROM DUAL WHERE FALSE; Empty set (0. 00 sec) See Also. SELECT

It consists of exactly one column named “dummy”, Whenever a simple value needs to be set or queried in SQL, it is time for the pseudo table DUAL to shine. One of the most popular use cases for this was the use of DUAL is a built-in table, useful because it is guaranteed to return only one row. This means DUAL may be used to get pseudo-columns such as 25 Feb 2016 A protip by ju about mysql, oracle, sql, select, and dual.

DUAL is in the schema of the user SYS but is accessible by the name DUAL to all users. It has one column, DUMMY, defined to be VARCHAR2(1), and contains one row with a value X. Selecting from the DUAL table is useful for computing a constant expression with the SELECT statement. Because DUAL has only one row, the constant is returned only once

If a user-defined table named DUAL exists, the Db2® server resolves a table reference to the user-defined table. Example 1 2016-09-14 · でも FROM DUAL を使えば where 句を書くことができる。 SELECT 1 as value FROM DUAL WHERE true; -- value: 1 が返される何に使うのだろうか。 1、select计算常量表达式、伪列等值. oracle内部处理使它只返回一行数据，而使用其它表时可能返回多个数据行。 2、查看当前用户. select user from dual; select count(*) from dual; 3、用做计算器. select 7*9*10-10 from dual； 4、调用系统函数（1）获得当前系统时间 듀얼 테이블(dual table)은 모든 오라클 데이터베이스 설치본에 기본으로 존재하는, 한 열로 이루어진 특별한 테이블이다. sysdate, user와 같은 의사컬럼 선택에 사용하는 것이 적절하다. 이 테이블은 'x' 값을 포함하는 dummy(더미)라 불리는 하나의 varchar2(1) 열이 있다.

FUNCTION TABLE. INPUTS. STROBE. OUTPUT. SELECT.