SQL COUNT DISTINCT example To exclude bothNULLvalues and duplicates, you use theCOUNT(DISTINCT column)as the following query: SELECTCOUNT(DISTINCTcountry)FROMcustomers;Code language:SQL (Structured Query Language)(sql) In this tutorial, you have learned how to use a various form of the SQL COUNT...
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( id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(20) ); Query OK, 0 rows affected (0.16 sec) -- 插入测试数据 INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C'); Query OK, 3 rows affected (1.05 sec) Records: 3 Duplicates: 0 Warnings: 0 -- 查看测试数据 SELECT * ...
SELECT COUNT(column_name) FROM table_name; Run code Powered By Variations of the syntax achieve different goals. The COUNT(*) syntax allows us to count the number of rows in a table The COUNT(DISTINCT column) syntax allows us to count the number of distinct values in a column The ...
We can count the number of distinct combinations across multiple columns using the COUNT DISTINCT clause and the CONCAT function in SQL. The CONCAT function allows us to concatenate two or more values into a single value which we can then use to compare and count. ...
---+ | status | count*) | +---+---+ | 0 | 407317 | | 1 | 4309 | +---+---+ 2 rows in set (0.17 sec) 因此范围查询索引失效的原因是查看数据量大并且需要回表。 优化方法:创建联合索引实现覆盖索引 alter tableboard_chute add index idx_status_create_time(status, _time); ...
DISTINCT With COUNT We can use SQLDISTINCTwith theCOUNT()function to count the number of unique rows. Let's look at an example. -- count the unique countries where customers are fromSELECTCOUNT(DISTINCTcountry)FROMCustomers; Run Code
The output is below. The first two columns count 28 rows, whereas the 3rdcolumn using DISTINCT counts 10 rows. This is 10 because column ProductName has only 10 unique values and the rest are duplicates. There is one more argument using which we can get the total number of rows in a ...
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(42000): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'scott.emp.deptno'; this is incompatible with sql_mode=only_full_group_by// 不查询部门号deptno的话就没什么问题,这时统计的是所有员工的数据mysql> select avg(sal), max(sal) from emp;+-...