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# 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.