170 lines
7.9 KiB
Markdown
170 lines
7.9 KiB
Markdown
---
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title: SQL Alter Table Statement
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---
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## SQL Guide - ALTER TABLE
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## Introduction
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This guide will introduce you to and attempt to explain some of the basics of the SQL alter table functions within a relational database.
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ALTER TABLE statement is used to add, remove, change datatype or rename columns. It can be also used to add or remove the constraints of the table.
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**IMPORTANT Safety Tip: ALWAYS backup your data before making changes!**
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We will be using MySQL for all examples throughout this freeCodeCamp SQL guide. The reasons for selecting MySQL are
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1) it is very commonly used on websites for the backend database,
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2) it's free, and is fun and easy to use.
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## Covered in this Guide
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We will use the tables created in the “CREATE TABLE” guide. Feel free to review that guide if you are not familiar with creating a table.
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* Adding a new column
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* Change column name
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* Change column definition
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* Remove a Column
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* Rename the Table
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## Alter Table:
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We will use the student table created in the “CREATE TABLE” guide.
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To view all column names and their description of student Table, we will use below command through out this guide.
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```sql
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desc student;
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```
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Result :
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| sat_score | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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* Adding a new column :
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Let's add a new column, emailAddress.
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```sql
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alter table student add column emailAddress varchar(30) NULL ;
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```
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Result :
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```sql
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desc student;
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```
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| sat_score | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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| emailAddress | NULL | YES | varchar(30) | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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* Change column name :
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Let's Change the column name sat_score to satScore .
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```sql
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alter table student change sat_score satScore int(4) NULL ;
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```
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Result :
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```sql
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desc student;
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```
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| satScore | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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| emailAddress | NULL | YES | varchar(30) | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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* Change column definition :
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Let's Change the emailAddress field size from varchar(30) to varchar(50).
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```sql
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alter table student change emailAddress emailAddress varchar(50) NULL ;
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```
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Result :
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```sql
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desc student;
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```
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| satScore | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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| emailAddress | NULL | YES | varchar(50) | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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* Remove a Column :
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Let's remove emailAddress column which we added earlier.
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```sql
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alter table student drop column emailAddress ;
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```
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Result :
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```sql
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desc student;
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```
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| satScore | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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* Rename the Table :
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Let's rename the table student to students.
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```sql
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alter table student rename to students ;
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```
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Result :
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we can also use describe command (alternative to desc) to see table description.
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```sql
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describe students
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```
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```text
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+--------------------+----------------------+--------------+-------------+------------+
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| column_name | column_default | is_nullable | column_type | column_key |
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+--------------------+----------------------+--------------+-------------+------------+
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| studentID | NULL | NO | int(11) | PRI |
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| FullName | NULL | YES | varchar(90) | |
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| studentIO | NULL | YES | int(3) | |
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| satScore | NULL | YES | int(4) | |
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| timeStampA | CURRENT TIMESTAMP | NO | timestamp | |
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| timeStampB | 0000-00-00 00:00:00 | NO | timestamp | |
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+--------------------+----------------------+--------------+-------------+------------+
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```
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There is much more that can be done, check the manual of your database management software to learn more.
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