Lesson 08( Table Management)

 

Alter Table 

It is used to change existing table structure such as adding or removing columns attributes etc.


Syntax-

alter table table_name action, [action2…..]


  1. Adding new column

  2. Drop a column

  3. Rename table


describe emp2;

We want to add a new column as dept.


alter table emp2;

→ add column dept int NOT NULL

→ after empid;


By default it will add at the end.


Drop field

Alter table table_name drop column_name


alter emp2

→ drop column dept;


Rename Table

Rename old table to new name


like

rename emp2 to emp6;


Drop Table

It will remove table


drop table table_name;


Truncate Table

It will clear the table data


truncate table emp6;

drop table emp6



Index in Mysql

An index helps in fast retrieval of data from tables. It is just like a index of book where you first search index to look for contents. All primary key column are in the primary index of the table automatically


Syntax-

Create index index_name on table_name column_name


Deleting Index

Create table emp_new3(eid, ename varchar(10));

Create index  i1 on emp_new3(eid);


To show index information

show index from emp_new3\G


Drop Index 

drop index i1 on emp_new3;

show index from emp_new3

(drop करने के बाद कमांड चलने पर empty set मिलेगा|)


If there is already st primary key there is already index is set.

Comments

Post a Comment

Popular posts from this blog

Lesson 07 (Primary Key & Foreign Key)

  Primary Key/Foreign Key Primary Key Combinagtion of column that uniquely identifies a row in table. Rules for Primary Key Primary key must contain unique value. Primary key can’t be null. There is only one primary key in table. Syntax- Col_name data_type PRIMARY KEY. like roll no in every class. We can define more than one column for primary key. PRIMARY KEY(COL1, COL2,......) PRIMARY KEY in existing table ALTER table(table_name) add PRIMARY KEY(primary key column) use college; create table emp11(empid int PRIMARY KEY, ename varchar(10)); describe emp11; create table emp12 (empid int, ename varchar(10),  → PRIMARY KEY(empid, ename); now you can see by describe command create table emp13 (empid int, ename varchar(10)); alter table emp13 → add PRIMARY KEY(empid); FOREIGN KEY It is a field in a table that matches another field of another table. It is used to maintain referential integrity. A table may have more than one foreign key and each fireign key in the child table may re...
Read more

Lesson 03 (Filter Data)

Filter Data with Where clause- Syntax- select              column1, column2…………… from              table1, table2…………………… where condition Data Filter करने के लिए हमें कुछ Conditional & Logical Operators का प्रयोग करना होता है - Conditional Operator =         Equal             बराबर  != or <>     Not Equal         बराबर नहीं  <         Less Than         बड़ा  >         More Than         छोटा  >=         More than Equal     बड़ा या बराबर  <=         Less than Equal     छोटा या बराबर  Logial Operator  An...
Read more

Lesson 05(Modifying Data)

  Modifying Data  (Insert, Update, Delete) Insert command का प्रयोग table में डाटा भरने के लिए किया जाता है | Syntax- insert into table(column1, column2,........) value(value1, value2….) select * from department; insert into department values (8, ‘ICE’); insert into department values (‘temp’, 6); ----error आ जाएगी | insert into department (dname, deptid) values (‘Temp’, 6) हो जायेगा | you can insert multiple rows insert into department values<enter> → (7, ‘A’), → (8, ‘B’);<enter> Insert with SELECT statement  It is used to insert data into table from another table. insert into table_name select col1, col2, …. from table_name; Update Statement Update statement is used to update existing data in a table . Syntax- update table_name set col1=exp1 col2=exp2 where condition We want to change deptname where deptid=7 in dept_temp. update department_temp → set deptname=’Applide Sc.’, → where deptid=7; select * from dept_temp; update dept_temp → set deptname=’test’; sel...
Read more