# 12 Basic SQL Commands for Beginners
## 1. Select
This `select` command is one of the simplest and most essential commands of SQL. As the name suggests, this command is used to select/retrieve the data from a database.
```
SELECT * FROM table;
```
This command can be broken down into two parts. The first part (`Select *`) specifies which columns need to be retrieved from a table, and the second part (`FROM table`) specifies the name of the table from which data needs to be retrieved.
## 2. Insert
We talked about retrieving the records from a table, but what about inserting a new record? The `insert` command is used to insert a new record into a table.
```
INSERT into table_name(column1, column2,...) VALUES (value1, value2,...);
```
We can specify the name of the table in which we want to insert a new record, column names, and values that need to be inserted.
The `VALUES` clause is used to specify that these are the values that need to be inserted.
## 3. Alias
In SQL, aliases are used to temporarily rename a table. An alias is created with the `as` keyword**.** We use the `as` command to provide a temporary name to an entity. We can give a temporary name to a table or any column inside the table.
Table alias syntax:
```
SELECT name, age FROM person as alias_name ;
```
Column alias syntax:
```
SELECT name as alias_name FROM person;
```
Aliases are also useful to avoid ambiguity errors when we join two tables and they both contain columns with the same name.
## 4. Where
We have seen that the `select` command is used to retrieve the data from a database, but what if we want to filter the records based on some conditions?
The `where` command allows you to apply some conditions on the `select` query to filter the data. We can use the `where` clause to limit the number of records in a table. The `where` clause is used at the end of the statement.
```
SELECT age, name FROM person p WHERE p.age > 10 ;
```
We can apply multiple conditions in a `where` clause by separating them with the `AND` operator.
```
SELECT age, name FROM person p WHERE p.age>10 AND p.age<20;
```
## 5. Order By
The `Order By` command is used to sort records based on specified columns. You can sort the records in ascending or descending order.
```
SELECT age, name FROM person ORDER BY age DESC;
```
We need to specify the names of the columns and the order: `ASC` for ascending and `DESC` for descending order.
## 6. Join
The `join` command is used to *combine* two or more tables present in a database based on the common column present in both tables.
We need to specify the name of the column based on which one we want to join both the tables.
```
SELECT s.name, s.rollNo, s.courseId FROM Student s JOIN Department d WHERE s.courseId = d.courseId ;
```
## 7. In
In SQL, the `In` operator is used to retrieve records based on the values present in the list of values.
```
SELECT name, salary FROM employee WHERE salary IN(20000, 25000, 30000);
```
The script above will retrieve all the records from the `employee` table where the salary of an employee will be either `20000`, `25000`, or `30000`.
We can also use the `NOT IN` clause to exclude the records that match the values present in the list.
## 8. Alter
The `alter` command is used to change the structure of the table. Using this `alter` command, we can `ADD` a new column to our table, `DROP` a column from the table, and also `MODIFY` the existing column present in the table.
Add column:
```
ALTER TABLE person ADD salary integer;
```
This script will add a column **salary** of integer type in the person table.
Drop column:
```
ALTER TABLE person DROP COLUMN age;
```
This script will drop the column named **age** from the person table.
Modify column:
```
ALTER TABLE person MODIFY salary float;
```
The script above will modify the type of the `salary` column from integer to float in the `person` table.
## 9. Update
After inserting the values in the table, wrong values could be inserted in some rows and need to be corrected.
In such cases, the `Update` command is used to update the values in specified rows. We can use the `where` clause to specify which records need to be updated. We can update the values of more than one column at a time.
```
UPDATE people SET age=25 WHERE name='John' ;
```
The query above will update the value of the `age` column of the record named ‘John’ to `25`.
## 10. Delete
The `Delete` command allows you to remove records from a table. You can delete specific records from a table by using the `where` clause along with the `delete` clause.
Its syntax is very simple and easy to use:
```
DELETE FROM person WHERE name='John' ;
```
The `delete` command must be used carefully because you cannot retrieve the data once the records are deleted.
## 11. Create Table
As the name suggests, the `create` command is used to create a new table in the SQL database. Its syntax is:
```
CREATE TABLE person(age integer, name varchar(50), salary integer);
```
You need to specify the name of the table (i.e. `person` in our example) and the column names that you want to be in your table, along with their respective data types.
## 12. Drop Table
The `drop` command is used to drop the table from a database. If we compare the `drop` command to the `delete` command, the `delete` command is used to delete the specific rows from a table. Meanwhile, the `drop` command is used to drop the whole table from a database.
Its syntax is :
```
DROP TABLE table_name ;
```
After executing this command, you will see that the whole table is removed from the database.
---
# Agent Instructions: Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://tvn.gitbook.io/sql/master.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.