Some points to be aware of in MySQL

A few common commands:

Select Database (); #查看当前所在的数据库.

Select User (); #查看当前登录的用户

Knowledge points that may be missed:

In the InnoDB storage engine, you must have a primary key in a table, and when you do not specify a field as the primary key, InnoDB selects the first non-empty and unique field as the key.

Foreign keys are unique to the InnoDB storage engine, other search engines do not have this feature, in fact, the foreign key is a combination of Python.

CREATE TABLE Table 2 Select required fields from table 1; #拷贝表1结构 + record to table 2

CREATE TABLE Table 2 Select required fields from table 1 where 1=2; #只拷贝表1的结构到表2

How to determine the three relationships of a table:

First, stand at the angle of a table and consider whether the table's multiple record objects can correspond to an object to join the table, that is, whether it is a multi------> one; second, stand in the perspective of another table, whether multiple objects of the table correspond to an object of another table, that is, whether it is more than one. If the two-time many-to-one is established, then the relationship between the two tables is many-to-many.

According to the above two comparisons, if only one is established, it is a pair-to-many relationship.

If you stand on two tables and think about it, it is a one-to-one relationship that only one object of a table corresponds to an object of another table. (Potential user form-------> Enrolment form)

Fields that are set as the primary key will default to their own non-empty, unique constraints.

Two types of characters;

char (NUMERIC): fixed-length string format, the length of the character is 0-255 (a character is also a char, using UTF-8 encoding when the use of three bytes), for the length of the stored characters is less than the value specified by char, the characters that are stored are populated, To the numeric length specified by char. Although space is wasted, access is highly efficient.

varchar (numeric): A variable length string format, that is, how long the character is deposited into the actual number of characters, but it is important to note here, for this kind of indefinite character format, you need a character to explain the length of the characters deposited, the last deposit format is: Data length description character + Actual character data. While this format saves space, access is inefficient and is recommended for less.

Some points to be aware of in MySQL