The following table will provide us the differences between PRIMARY KEY and UNIQUE constraint −PRIMARY KEYUNIQUE Constraint1. Only one Primary key can be created on a table.1. More than one UNIQUE Constraints can be added to a table.2. Primary key creates clustered index by default.2. UNIQUE Constraint creates a non-clustered index by default.3. We cannot insert null values in the column which is defined as PRIMARY KEY.3. We can insert null values in the column having a UNIQUE constraint.

Actually, both of these data types in MySQL store strings and can be set with a maximum length. The use of these data types purely depends on the need. Followings are some points which will make us clear that when one should CHAR and when VARCHAR −Suppose if we have fixed size of data, such as flags of “Y” and “N” then it is better to use CHAR rather than VARCHAR. It is because 1 byte of length prefix also used with VARCHAR. In other words, for the above kind of data, CHAR will store the only 1byte which is ... Read More

CHAR and VARCHAR are both ASCII character data types and almost same but they are different at the stage of storing and retrieving the data from the database. Following are some important differences between CHAR and VARCHAR in MySQL −CHAR Data TypeVARCHAR Data TypeIts full name is CHARACTERIts full name is VARIABLE CHARACTERIt stores values in fixed lengths and are padded with space characters to match the specified lengthVARCHAR stores values in variable length along with 1-byte or 2-byte length prefix and are not padded with any charactersIt can hold a maximum of 255 characters.It can hold a maximum of 65, ... Read More

In MySQL, both CHAR and NCHAR are ASCII character data types used for storing text data, but they differ significantly in terms of storage, data representation, and performance. CHAR and NCHAR columns can have different collations, determining how strings are compared and sorted. The CHAR type typically uses the collation associated with its specified character set. On the other hand, NCHAR is intended for Unicode data and typically uses a collation that can handle Unicode characters, ensuring proper sorting and comparison. Understanding 'CHAR' in MySQL The CHAR data type is primarily used to store ASCII character data. It is a ... Read More

When two tables are connected with Foreign key and data in the parent table is deleted, for which record exists in child table too, then followings are the ways to maintain data integrity −On Delete CascadeThis option will remove the record from child table too if that value of the foreign key is deleted from the main table.On Delete Null This option will set all the values in that record of child table as NULL, for which the value of the foreign key is deleted from the main table.

We can disable foreign key checks with the help of the following statement −mysql> Set foreign_key_checks = 0; Query OK, 0 rows affected (0.00 sec)And we can enable it with the help of the following statement −mysql> Set foreign_key_checks = 1; Query OK, 0 rows affected (0.00 sec)Some benefits of disabling foreign key checks are as follows −After disabling foreign key checks we can load data into parent and child table in any order. Otherwise, we must have to load the data first in the parent table and then in the child table.Without disabling foreign key checks we cannot drop ... Read More

We can remove FOREIGN KEY constraint from a column of an existing table by using DROP keyword along with ALTER TABLE statement. Syntax ALTER TABLE table_name DROP FOREIGN KEY constraint_name Here constraint name is the name of foreign key constraint which we applied while creating the table. If no constraint name is specified then MySQL will provide constraint name which can be checked by SHOW CREATE TABLE statement. Example The following query will delete the FOREIGN KEY constraint from ‘orders’ table − mysql> Alter table orders DROP FOREIGN KEY orders_ibfk_1; Query OK, 0 rows affected (0.22 sec) ... Read More

On specifying ZEROFILL for a numeric column, MYSQL automatically pads the displayed value of the field with zeros up to the display width specified in the column definition.For example, we create a table showzerofill and insert the values as follows −mysql> Create Table showzerofill(Val1 INT(5) ZEROFILL, Val2 INT(5)); Query OK, 0 rows affected (0.09 sec) mysql> Insert into showzerofill(Val1, Val2) values(1, 1>, , , , ; Query OK, 5 rows affected (0.03 sec) Records: 5 Duplicates: 0 Warnings: 0Now we can easily understand the effect of ZEROFILL on the values of column Val1.ZEROFILL padded zeros in the number up ... Read More

While deleting the row from the parent table, if the data of that row is used in the child table then MySQL will throw an error because of the failure of FOREIGN KEY constraint. It can be understood with the example of two tables named ‘customer’ and ‘orders’. Here, ‘customer’ is the parent table and ‘orders’ is the child table. We cannot delete a row, that is used in child table ‘orders’, from the ‘customer’ table. It can be demonstrated by deleting the values from the parent table as follows −mysql> Select * from Customer; +----+--------+ | id | name ... Read More

Actually, foreign keys enforce referential integrity that helps us to maintain the consistency and integrity of the data automatically. It can be understood with the example of two tables named ‘customer’ and ‘orders’. Here, ‘customer’ is the parent table and ‘orders’ is the child table. We cannot create an order for a non-existent customer. It can be demonstrated by inserting the values in both the tables as follows −mysql> Select * from Customer; +----+--------+ | id | name   | +----+--------+ | 1  | Gaurav | | 2  | Raman  | | 3  | Harshit| | 4  | Aarav  | ... Read More