When adding the COUNT function with a WHERE clause, a first assumption would be to simply add it directly inline like we would with any other filter value. For example, with the HAVING clause as mentioned earlier. On the contrary, T-SQL will not allow an aggregate within the WHERE clause ...
COUNT(*)和COUNT(列)在 SQL 查询中用于不同的目的。COUNT(*)用于统计表中的行数,而COUNT(列)用于统计指定列中非 NULL 值的数量。在性能上,两者的差异通常不明显,但在特定情况下(如列是主键或包含索引时),COUNT(列)可能利用索引优化查询速度。然而,这取决于数据库的具体实现和查询优化器的行为。
we accept them as facts. Sometimes the claims are valid for a given context. Sometimes people are wrong about a given topic. I’ve held my share of false beliefs until someone pointed out my error. Early on, I assumed SQL obtained row counts from statistics. Looking back, I have no ide...
Simple SQL COUNT(*) example To get the number of orders in theorderstable, you use theCOUNT(*)function as follows: SELECTCOUNT(*)FROMorders;Code language:SQL (Structured Query Language)(sql) The pending order is the order whose shipped date isNULL. To get the number of pending orders, ...
Let’s see an example of using an IF condition inside the COUNT function. We can place an IF expression inside the COUNT function and set the value to NULL for false condition and any non-null value for the true condition. Every NON NULL value would be counted as a single row w.r.t...
ExampleGet your own SQL Server SELECTCOUNT(ProductID) FROMProducts; Try it Yourself » Note:NULL values are not counted. AVG() Example The following SQL statement finds the average price of all products: Example SELECTAVG(Price) FROMProducts; ...
SQL COUNT() FAQs Can COUNT() count only specific rows? Yes, you can use a WHERE clause with COUNT() to count only rows that meet specific criteria. For example: SELECT COUNT(*) FROM employees WHERE department = 'Sales'; Powered By What is the difference between COUNT(*) and COUNT...
For MyISAM tables,COUNT(*)is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved, and there is no WHERE clause. For example: mysql> SELECT COUNT(*) FROM student; This optimization only applies to MyISAM tables, because an exact row cou...
SELECT COUNT( ALL val ) FROM items; Code language: SQL (Structured Query Language) (sql) B) Simple Oracle COUNT() example The following example returns the number of rows in the products table: SELECT COUNT(*) FROM products; Code language: SQL (Structured Query Language) (sql) C) Oracle...
上面我们聊完了结果集上的差异,下面我们来看看性能、SQL 语句底层运行上的差异。 InnoDB count(field) 当前列如果有索引,则使用索引进行计数,如果没有索引则进行全表扫描。 实践 // 没有索引 进行全表扫描 explain SELECT COUNT(class_no) FROM student; ...