Altering Database Tables: Drop Unique Constraint and Add Primary Key : Alter Table « Table Index « SQL / MySQL






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Altering Database Tables: Drop Unique Constraint and Add Primary Key

   
/*

mysql> CREATE TABLE Employee (
    ->    Name    VARCHAR(50) PRIMARY KEY NOT NULL,
    ->    PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone',
    ->    Age     INT CHECK (Age BETWEEN 20 and 30));
Query OK, 0 rows affected (0.04 sec)

mysql> Describe Employee;
+---------+-------------+------+-----+---------------+-------+
| Field   | Type        | Null | Key | Default       | Extra |
+---------+-------------+------+-----+---------------+-------+
| Name    | varchar(50) |      | PRI |               |       |
| PhoneNo | varchar(15) | YES  |     | Unknown Phone |       |
| Age     | int(11)     | YES  |     | NULL          |       |
+---------+-------------+------+-----+---------------+-------+
3 rows in set (0.01 sec)

mysql> Select * from Employee;
+----------+---------------+------+
| Name     | PhoneNo       | Age  |
+----------+---------------+------+
| John Doe | Unknown Phone |   31 |
+----------+---------------+------+
1 row in set (0.00 sec)

mysql> ALTER TABLE Employee DROP INDEX Name;
ERROR 1091 (42000): Can't DROP 'Name'; check that column/key exists
mysql> ALTER TABLE Employee ADD PRIMARY KEY (Name);
ERROR 1068 (42000): Multiple primary key defined
mysql> Describe Employee;
+---------+-------------+------+-----+---------------+-------+
| Field   | Type        | Null | Key | Default       | Extra |
+---------+-------------+------+-----+---------------+-------+
| Name    | varchar(50) |      | PRI |               |       |
| PhoneNo | varchar(15) | YES  |     | Unknown Phone |       |
| Age     | int(11)     | YES  |     | NULL          |       |
+---------+-------------+------+-----+---------------+-------+
3 rows in set (0.00 sec)

*/
Drop TABLE Employee;
       
CREATE TABLE Employee (
   Name    VARCHAR(50) PRIMARY KEY NOT NULL, 
   PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone',
   Age     INT CHECK (Age BETWEEN 20 and 30));

Describe Employee;

INSERT INTO Employee (Name, Phone, Age) VALUES ('Joe Wang', '666 2323', 26);
INSERT INTO Employee (Name, Age) VALUES ('John Doe', 31);

Select * from Employee;
  
  
ALTER TABLE Employee DROP INDEX Name;
ALTER TABLE Employee ADD PRIMARY KEY (Name);

Describe Employee;








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16.Changing the Table Design (ALTER TABLE)
17.Use ALTER TABLE to change the column type
18.The Effect of ALTER TABLE on Null and Default Value Attributes
19.Create the table and then set the initial sequence value with ALTER TABLE
20.Alter table to set the primary key start
21.ALTER TABLE statement modifies the table accordingly
22.Alter table to add primary key, add new column, change column, drop column
23.Using ALTER TABLE command to add the names_num column to the cust_names table
24.Two ALTER TABLE statements can be combined
25.Alter table to add foreign key
26.Syntax for Altering a Column
27.Syntax for altering table and delete a Column
28.Storing Altered Character Strings
29.To drop a default value, use ALTER col_name DROP DEFAULT:
30.ALTER column to BINARY
31.Use the asterisk to try matching against the name ian or the alternative spelling, iain.
32.Specifying Multiple Alterations
33.Alter table to add an index
34.Alter table to add unique index
35.Alter table to Delete an Index
36.Alter table to add index
37.To drop an index with ALTER TABLE
38.Reduce the number of significant characters per index in the index to the first 16 characters
39.Add IGNORE to the statement, then use SELECT to have a look at the table contents to see how the duplicates ha
40.Change j from INT to BIGINT
41.MODIFY statement should specify the null and default value attributes explicitly
42.Redefine i to be NOT NULL, then try again
43.Change column definition