1.What is the purpose of using DISTINCT on multiple columns in SQL? DISTINCT on multiple columns eliminates rows where all selected fields are identical, ensuring unique combinations of the specified columns in the result set. 2.How does DISTINCT work with multiple columns? When used with multipl...
SELECT DISTINCT vs SELECT TheSELECT DISTINCTstatement is used when you want to return only unique (distinct) values in the result set. Whereas, a regularSELECTstatement without theDISTINCTkeyword retrieves all rows from the specified columns, including duplicate values. Let's look at an example. ...
SELECT DISTINCT CONCAT(first_name, ' ', last_name) AS full_name FROM employees; 在这个查询中,CONCAT(first_name, ' ', last_name)用于合并first_name和last_name列,并在它们之间添加一个空格。DISTINCT关键字确保查询结果中的每一行都是唯一的。 解释查询结果并指出注意事项: 查询结果将是一个包含唯一...
mysql> select distinct tiny_column from big_table limit 2; mysql> -- Returns the unique combinations of values from multiple columns. mysql> select distinct tiny_column, int_column from big_table limit 2; distinct可以和聚合函数(通常是count函数)一同使用,count(disitnct)用于计算出一个列或多个...
union:对两个结果集进行并集操作,不包括重复行,相当于distinct,同时进行默认规则的排序; union all:对两个结果集进行并集操作,包括重复行,不进行排序; union因为要进行重复值扫描,所以在结果集庞大的情况下,效率极低,因此建议使用union all。 若结果集去重是强需求,则在应用程序代码上进行去重,因为数据库资源要比应...
The output fromEXPLAINshowsALLin thetypecolumn when MySQL uses afull table scanto resolve a query. 慢SQL 分析与优化 https://mp.weixin.qq.com/s/CaSVhAJgycjjbCxAkII2ZA 从系统设计角度看,一个系统从设计搭建到数据逐步增长,SQL 执行效率可能会出现劣化,为继续支撑业务发展,我们需要对慢 SQL 进行分析和...
Distinct count with multiple columns distinct vs group by performance wise Divide time into Morning , After noon , Evening and Night Do I need to INCLUDE the primary key in an index or not? Do not select the last row Does anyone know how to find a period character in a string? Does ca...
SQL_MAX_COLUMNS_IN_INDEX SQL_MAX_COLUMNS_IN_ORDER_BY SQL_MAX_COLUMNS_IN_SELECT SQL_MAX_COLUMNS_IN_TABLE SQL_MAX_CURSOR_NAME_LEN SQL_MAX_IDENTIFIER_LEN SQL_MAX_INDEX_SIZE SQL_MAX_PROCEDURE_NAME_LEN SQL_MAX_ROW_SIZE SQL_MAX_ROW_SIZE_INCLUDES_LONG SQL_MAX_SCHEMA_NAME_LEN SQL_MAX_STATEM...
解决方法:使用Query洞察排查当前Query是否在同一时间执行了TRUNCATE或DROP操作,详情请参见Query洞察。 ERRCODE_FEATURE_NOT_SUPPORTED或者Unsupported Feature 报错:Feature not supported: insert into parent table 问题原因:不支持直接写入数据至分区父表。 解决方法:SQL方式执行INSERT时,需要将数据写入指定的分区子表。详情...
> SELECT coalesce('6.1', 5); Error: CAST_INVALID_INPUT -- The least common type between a DECIMAL and a STRING is a DOUBLE > SELECT typeof(coalesce(1BD, '6')); DOUBLE -- Two distinct explicit collations result in an error > SELECT collation(coalesce('hello' COLLATE UTF8_BINARY, '...