MySQL - the difference between count (field), count (primary key), count (1), count (*)

count( Field )、count( Primary key )、count(1)、count(*) The difference between

• count() It's an aggregate function , Judge the returned result set line by line , If count The field value corresponding to the parameter in the function is not Null, Add... To the cumulative value 1, Otherwise, we will not add , Finally return the cumulative value .
• count(*)、count(1) and count( Primary key id) All return the total number of rows of the result set that meet the conditions ; and count( Field ）, Then the value in the returned field is not Null The total number of rows of .

---

With weibo_user Table as an example , There are 99 Data ,created_at The field value is not Null Data are available. 6 strip

---

• count(*) Query all data

---

• count(id) Query all data

---

• count(1) Query all data

---

• count(create_at) Query only 6 Data , Values are not for NULL The data of

---

Execution efficiency

• If the data table has no primary key , that count(1) Than count() fast ;
• If there is a primary key , That's the primary key count It's better than count() Be quick ;
• If the table has only one field ,count(*) It's the fastest .