MySQL -count: the difference between count (1), count (*), and count (column name)

Come to the conclusion first

On the effect of execution ：

• count(*) It includes all the columns , Equivalent to the number of lines , When it comes to statistical results , Will not be ignored as NULL Value .
• count(1) use 1 Represents the line of code , When it comes to statistical results , Will not be ignored as NULL Value .
• count( Name ) Only include the column of the list , When it comes to statistical results , Will ignore null column values （ Null here does not mean an empty string or 0, It means null） Count of , That is, the value of a field is NULL when , No statistics .

In terms of execution efficiency ：

• The column name is primary key ,count( Name ) than count(1) fast
• Column name is not primary key ,count(1) than count( Name ) fast
• If the table has multiple columns and no primary key , be count(1） Is superior to the execution efficiency count（*）
• If there is a primary key , be select count（ Primary key ） The efficiency of execution is optimal
• If the table has only one field , be select count（*） The optimal .

make a concrete analysis

1. count(1) and count(*)

In terms of execution plan ,count(1) and count(*) The effect is the same .

When the table has more data , After analyzing the table , Use count(1) It's better than using count(*) It takes a lot of time ！

When the amount of data is 1W Within ,count(1) than count(*) We need less time , But not much .

If count(1) When it's a clustered index , That must be count(1) fast , But the difference is very small .

because count(), It will automatically optimize the field assigned to . So there's no need to count(1), Use count(),sql It will help you optimize

therefore ： When there is a clustered index count(1) and count(*) Basically no difference ！

2. count(1) and count( Field )

The main difference between the two is ：
count(1) Will count all the records in the table , The containing field is null The record of .
count( Field ) Count the number of times the field appears in the table , The ignored field is null The situation of . That is to say, do not count the fields as null The record of .