Table of contents
overview
There is another type of SQL function, called aggregation (or aggregation, grouping) function, which is a function that summarizes a set of data. The input is a set of data, and the output is a single value.
Illustration:
SUM()
Meaning: Returns the total number of numeric columns.
适用:PostGreSql、MySQL、ORACLE 、Microsoft SQL Server
grammar:
SELECT AVG(column_name) FROM table_name
AVG()
Returns the average of a numeric column.
适用:PostGreSql、 ORACLE 、Microsoft SQL Server、MySQL
grammar:
SELECT AVG(column_name) FROM table_name
COUNT()
Meaning: Returns the number of rows matching the specified criteria.
适用:PostGreSql、 ORACLE 、Microsoft SQL Server、MySQL
Notice:
- COUNT(*) returns the total number of records in the table, applicable to any data type .
- COUNT(expr) returns the total number of records where expr is not empty .
- COUNT(*) counts rows with NULL values, but COUNT(column name) does not count rows with NULL values in this column.
grammar:
--COUNT(column_name) 函数返回指定列的值的数目(NULL 不计入):
SELECT COUNT(column_name) FROM table_name;
--COUNT(*) 函数返回表中的记录数(无法用于 Microsoft Access):
SELECT COUNT(DISTINCT column_name) FROM table_name;
--COUNT(DISTINCT column_name) 函数返回指定列的不同值的数目:
SELECT COUNT(*) FROM table_name;
MAX()
Meaning: returns the maximum value of the specified column
适用:PostGreSql、 ORACLE 、Microsoft SQL Server、MySQL
grammar:
SELECT MAX(column_name) FROM table_name;
MIN()
Meaning: Returns the minimum value of the specified column.
适用:PostGreSql、 ORACLE 、Microsoft SQL Server、MySQL
grammar:
SELECT MIN(column_name) FROM table_name;
ARRAY_AGG()
Meaning: Returns an array consisting of all values in the field within the group.
Applicable: PostGreSql,
grammar:
SELECT ARRAY_AGG(column_name) FROM table_name;