复制 create tablet1(id1 int,id2varchar(64));insert into t1values(1,’nanjing’),(1,’suzhou’),(2,’xingtai’),(2,’shijiazhuang’);select id1,string_agg(id2,’,’)group by id1;id1|string_agg---1|nanjing,suzhou2|xingtai,shijiazhuang array_agg函数和string_agg函数类似,最主要的区别...
Discover how to retrieve the first row per group in PostgreSQL using the row_number function. Learn how to identify the initial event for each user on a specific day with PostgreSQL queries.
SQL> SELECT last name, dept_id, salary 2 FROM s_emp 3 ORDER BY dept_id, salary DESC; 八、限制选取行: SELECT expr FROM table [WHERE condition(s)] [ORDER BY expr]; 例1: SQL> SELECT first_name, last_name, start_date 2 FROM s_emp 3 WHERE start_date BETWEEN ’09-may-91’ 4 A...
[ GROUP BY grouping_element [, ...] ] [ HAVING condition [, ...] ] [ WINDOW window_name AS ( window_definition ) [, ...] ] [ { UNION | INTERSECT | EXCEPT } [ ALL | DISTINCT ] select ] [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ...
SELECT * FROM users ORDER BY first_name, last_name ASC; 在应用场景中,当需要根据多个列进行查询,并且查询结果需要按照指定的顺序返回时,使用复合索引可以提高查询性能。 腾讯云提供了云数据库 PostgreSQL,是一种高性能、可扩展的云原生数据库服务。它提供了自动备份、容灾、监控等功能,适用于各种规模的应用场...
PARTITION BY选项用于定义分区,作用类似于GROUP BY的分组。如果指定了分区选项,窗口函数将会分别针对每个分区单独进行分析;如果省略分区选项,所有的数据作为一个整体进行分析,上文中的示例就是如此。 以下语句按照部门进行分组,分析每个部门的平均月薪: SELECTfirst_name,last_name,department_id,salary,AVG(salary)OVER(...
SELECT * FROM employee WHERE (department, salary) IN ( SELECT department, MAX(salary) FROM employee GROUP BY department ) ORDER BY department; 使用窗口函数 row_number() WITH ranked_employees AS ( SELECT ROW_NUMBER() OVER ( PARTITION BY department ORDER BY salary DESC ...
// src/include/storage/lockdefs.h#define NoLock 0#define AccessShareLock 1/* SELECT */#define RowShareLock 2/* SELECT FOR UPDATE/FOR SHARE */#define RowExclusiveLock 3/* INSERT, UPDATE, DELETE */#define ShareUpdateExclusiveLock 4/* VACUUM (non-FULL),ANALYZE, CREATE INDEX CONCURRENTLY *...
在第二个查询里,我们不能写成SELECT * FROM test1 GROUP BY x, 因为列y里没有哪个值可以和每个组相关联起来。被分组的列可以在选择列表中引用是因为它们在每个组都有单一的值。通常,如果一个表被分了组,那么没有在GROUP BY中列出的列都不能被引用,除非在聚集表达式中被引用。 一个用聚集表达式的例子是: =...
WHERE(department,salary)IN(SELECT department,MAX(salary)FROM employee GROUP BY department)ORDER BY department; 使用窗口函数 row_number() WITHranked_employeesAS(SELECTROW_NUMBER()OVER(PARTITIONBYdepartmentORDERBYsalaryDESC)ASrn,*FROMemployee)SELECT*FROMranked_employeesWHERErn=1ORDERBYdepartment; ...