InnoDBhandlesSELECT COUNT(*)andSELECT COUNT(1)operations in the same way. There is no performance difference. 简单的来说就是,InnoDB下 count(*) 等价于 count(1) 既然会自动走索引,那么上面那个所谓的速度排序还觉得对吗? count(*)的性能跟数据量有很大的关系
The like operator uses underscore (_) as a wildcard for a single character and the percent sign (%) as the wildcard for any number of characters—like ? and * in Excels countif. Excel: =COUNTIF(Ax:Ay, "Marvin*") SQL: COUNT(CASE WHEN A LIKE 'Marvin%' THEN 1 END)...
As can be seen from the user-defined function, it loops through each character in the input string (WHILE @vIndex <= @vInputLength) and compares the string characer against the input search characer (IF SUBSTRING(@pInput, @vIndex, 1) = @pSearchChar). If they are the same, the cou...
Counts the number of times that a regular expression pattern is matched in a string. syntaxsql REGEXP_COUNT(string_expression,pattern_expression[ , start [ , flags ] ] ) Arguments string_expression An expression of a character string.
The COUNT function is commonly run with the * (star or asterisk) character within the parentheses. That is the first column in this query below. It is run again for each column individually. Finally, the SQL query is run with the constant values 1 and “hello”. ...
The COUNT() function is an aggregate function that is used to find the number of values in the specified column excluding NULL values. It can be applied on numeric, character, or date values. Syntax: SELECT COUNT([ALL | DISTINCT] expression | column_name) FROM table_name [WHERE condition...
Postgresql支持变长参数传递，参数被自动转换为数据传入函数体中，类似C语言的可变参数：int sum(int num...
`item_code` varchar(36) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '考核项目编码', `item_name` varchar(64) COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '考核项目名称', `item_value` varchar(36) COLLATE utf8mb4_general_ci DEFAULT NULL COMMENT '考核结果',...
选择刚才备份的sql脚本执行即可。 4.多表查询 USE test; -- 创建部门表 CREATE TABLE dept ( id INT PRIMARY KEY AUTO_INCREMENT, NAME VARCHAR(20) ); /* 解决插入数据乱码问题 */ SHOW FULL COLUMNS FROM dept;-- 查看所有字段的编码集 ALTER TABLE dept CONVERT TO CHARACTER SET utf8; -- 将所有字...
SQL COUNT rows in a table In the following example, an asterisk character ( * ) is used followed by the SQL COUNT() which indicates all the rows of the table even if there is any NULL value. Sample table: orders ORD_NUM ORD_AMOUNT ADVANCE_AMOUNT ORD_DATE CUST_CODE AGENT_CODE ORD_DE...