The count () function in MySQL uses the

Sometimes it is considered that count (*) is slower than count (1) or count (column name) and is actually handled in a sub-case.

Like what:

---initialization statements

Create a table and insert the data:

Create table test2 (id BIGINT PRIMARY key, name varchar)engine=innodb;

insert Into Test2 (id,name) values (1, NULL);

insert into test2 (id,name) values ( Span class= "Hljs-number" >2, ' name1 ');

insert Into Test2 (id,name) values (3, " Name2 ')

Execute the following SELECT statement:

select count (*) from test2;  //Result: 3

select count (ID) from test2;//Result: 3

select count (name) from test2;//Result: 2

select count (name) from test2 where name is null; Result: 0

count (1) refers to the first field of the table rather than the count of 1. If the first field does not have an index, his efficiency is very low;

), in the above count (column name)

in query mode, if the specified column is limited to non-null, MySQL then converts the above expression to count () for querying. So if you want

to query the size of the data, it is recommended that count (*) be used in MySQL to execute, meaning and fast.

The count () function in MySQL uses