MySql data integrity constraints

Entity integrity (primary key constraint unique constraint self-growth)
domain integrity (non-null constraint default value constraint)
referential integrity (foreign key constraint)

Constraint
primary key constraint
Primary key constraint:
The data in the column that is constrained by the primary key constraint cannot be NULL, and cannot be repeated [not empty, unique]

Note: There can only be one primary key constraint
DROP TABLE dept in each table ;

• When creating the table, add the primary key constraint
  CREATE TABLE dept(
  deptno INT PRIMARY KEY,-add the primary key constraint
  dname VARCHAR(10),
  address VARCHAR(100)
  ) to the deptno field ;
  INSERT INTO dept VALUES(20,'Research Department x' ,'Dallas'); – The addition is successful because it does not violate the rules of primary key constraints.
  INSERT INTO dept VALUES(20,'Research Department x','Dallas'); – The addition fails because 20 is repeated

• Delete the primary key constraint
  ALTER TABLE dept DROP PRIMARY KEY;

• Set the primary key constraint for the created table
  ALTER TABLE emp CHANGE empno empno INT(11) PRIMARY KEY;

• Primary key increment
  DROP TABLE dept;
  CREATE TABLE dept(
  deptno INT PRIMARY KEY AUTO_INCREMENT, – Add a primary key constraint to the deptno field
  dname VARCHAR(10),
  address VARCHAR(100)
  );

• 0 and null use self-increasing value
  INSERT INTO dept VALUES(0,'Research Department x','Dallas');
  INSERT INTO dept VALUES(null,'Research Department x','Dallas');
  INSERT INTO dept (dname, address) VALUES('Research Department x','Dallas');

• The specific value uses the specific value
  INSERT INTO dept VALUES (999,'Research Department x','Dallas'); the
  only constraint
  is the uniquely constrained field, the value cannot be repeated.
  DROP TABLE dept;

• When creating a table, add a unique constraint
  CREATE TABLE dept(
  deptno INT PRIMARY KEY,
  -add a primary key constraint to the deptno field dname VARCHAR(10) NOT NULL,-add a non-null constraint to the DNAME field,
  address VARCHAR(100) UNIQUE NOT NULL-add unique and non-null constraints to address
  );

• Delete the unique constraint: The unique constraint is a kind of index, so use the syntax of deleting the index to delete the unique constraint.
  ALTER TABLE dept DROP INDEX address;
  or
  ALTER TABLE emp CHANGE empno empno INT(11).

• Add a unique constraint to the created table
  ALTER TABLE dept CHANGE dname dname VARCHAR(10) NOT NULL UNIQUE;
  non
  -null constraint The value of the column constrained by non-null cannot be NULL
  DROP TABLE dept;

• When creating the table, add a non-null constraint
  CREATE TABLE dept(
  deptno INT PRIMARY KEY,
  -add a primary key constraint to the deptno field dname VARCHAR(10) NOT NULL,-add a non-null constraint to the DNAME field,
  address VARCHAR(100)
  ) ;

INSERT INTO dept VALUES(10,'xxx','Dallas'); – Successful
INSERT INTO dept VALUES(20,'YYY','Dallas'); – Successful
INSERT INTO dept VALUES(30,NULL,'Dallas'); --Failed because of non-empty constraint violation

• Delete the non-empty constraint
  ALTER TABLE dept CHANGE dname dname VARCHAR(10);
• Add a non-null constraint to the created table
  ALTER TABLE dept CHANGE dname dname VARCHAR(10) NOT NULL;
  Default value
  Set the default value constraint field, if not assigned, use the default value
  DROP TABLE dept;
• When creating the table, add the default value constraint
  CREATE TABLE dept(
  deptno INT PRIMARY KEY,
  -add the primary key constraint to the deptno field dname VARCHAR(10) NOT NULL,-add the non-null constraint to the DNAME field,
  address VARCHAR(100) UNIQUE NOT NULL DEFAULT'Shanghai'-add unique and non-null constraints to address
  );
  INSERT INTO dept VALUES(10,'xxx','Dallas');
  INSERT INTO dept (deptno,dname) VALUES(210,'xxxY');
  INSERT INTO dept VALUES(30,'xxx','Dallas');
• Delete the default value constraint
  ALTER TABLE dept CHANGE address address VARCHAR(10);
• Add default value constraint
  ALTER TABLE dept CHANGE address address VARCHAR(10) NOT NULL DEFAULT'Shanghai';
  foreign key constraint

Is the constraint between the two tables. For example, use the a field in the A table to constrain the data in the b field in the B table, the A table is called the primary table [parent table], and the B table is called the secondary table [child table]. The primary key must be used to constrain the secondary table in the primary table.

DROP TABLE dept;
DROP TABLE emp;
CREATE TABLE dept(
deptno INT PRIMARY KEY, - Add a primary key constraint to the deptno column
dname VARCHAR(10),
address VARCHAR(100)
);
Add a foreign key constraint
CREATE TABLE emp(
empno INT,
deptnoX INT,
CONSTRAINT fk_dept_emp FOREIGN KEY (deptnoX) REFERENCES dept(deptno)
)

INSERT INTO dept VALUES(10,'xxx','Dallas');
INSERT INTO dept VALUES(30,'xxx','Dallas');

INSERT INTO emp VALUES(10,10);
INSERT INTO emp VALUES(30,30);
INSERT INTO emp VALUES(70,70);-Adding failed because there is no department 70 in the dept table of the main table

• Delete foreign key constraints
  ALTER TABLE tableName DROP FOREIGN KEY fk_name;
  ALTER TABLE emp DROP FOREIGN KEY fk_dept_emp;

Function
character function

• CHARSET(str) returns the string character set
• CONCAT (string2 [,… ]) connection string
• INSTR (string, substring) returns the position where the substring appears in the string, without returning 0
• UCASE (string2) converted to uppercase
• LCASE (string2) converted to lowercase
• LEFT (string2, length) Take length characters from the left in string2
• LENGTH (string) string length
• REPLACE (str ,search_str ,replace_str) Replace with replace_str in str-search_str
• STRCMP (string1, string2) compares the size of two strings character by character
• SUBSTRING (str, position [,length ]) Starting from the position of str, take length characters
• LTRIM (string2) RTRIM (string2) trim removes leading or trailing spaces

Mathematical function

• ABS (number2) absolute value
• BIN (decimal_number) decimal to binary
• CEILING (number2) round up
• CONV(number2,from_base,to_base) base conversion
• FLOOR (number2) round down
• FORMAT (number,decimal_places) to hexadecimal
• LEAST (number, number2 [,…]) find the minimum
• MOD (numerator ,denominator ) 求余
• RAND([seed]) RAND([seed])

Date and time functions

• ADDTIME (date2, time_interval) add time_interval to date2
• CURRENT_DATE () current date
• CURRENT_TIME ( )
• CURRENT_TIMESTAMP () Current time and
  current timestamp
• DATE (datetime) returns the date part of datetime
• DATE_ADD (date2, INTERVAL d_value d_type) add date or time to date2
• DATE_SUB (date2, INTERVAL d_value d_type) subtract a time from date2
• DATEDIFF (date1, date2) the difference between two dates
• NOW () current time
• YEAR|Month|DATE (datetime ) 年月日

System Information Function

Encryption function
Function role

• Password(str) is generally used to encrypt the user's password
• Md5(str) Encrypt common data

Other functions

• The CONVERT(S USING CS) function converts the character set of the string S into CS.
• Convert the encoding of the
  string'ABC' to GBK. SELECT CHARSET('ABC'),CHARSET(CONVERT('ABC' USING'GBK'));