SQL DISTINCT keyword: delete duplicate records

The DISTINCT keyword in SQL is used to return unique values in the output. It eliminates all the duplicate records from the result set.

The basic syntax of DISTINCT is:

SELECT DISTINCT column_name 
FROM table_name;

Example:

Suppose we have a table Employees with the following data:

ID	Name	Department
1	John Doe	Sales
2	Jane Doe	Marketing
3	Mike Smith	Sales
4	Alice Johnson	Sales
5	Bob Johnson	Marketing

If we want to find out all the unique departments in the table, we can use the DISTINCT keyword:

SELECT DISTINCT Department 
FROM Employees;

This query will return:

Sales
Marketing

You can also use the DISTINCT keyword with multiple columns. The query will then return unique combinations of those columns.

For example:

SELECT DISTINCT Name, Department 
FROM Employees;

This query will return all the unique Name and Department combinations from the Employees table.

It's important to note that DISTINCT applies to the combination of all columns listed after the DISTINCT keyword, not each column individually.

Finally, remember that the DISTINCT keyword can impact performance, as the database needs to sort and compare the result set to remove duplicates. Use it wisely.

Using DISTINCT to Identify Duplicate Records:
- Description: DISTINCT is used to retrieve unique values from a single column.
- Code Example:
```
SELECT DISTINCT ColumnName
FROM TableName;
```

SQL DELETE Duplicate Records with DISTINCT:

Description: DELETE statement with DISTINCT to remove duplicate records.

Code Example:

DELETE FROM TableName
WHERE ColumnName IN (
    SELECT DISTINCT ColumnName
    FROM TableName
    HAVING COUNT(ColumnName) > 1
);

Removing Duplicate Rows with SQL DISTINCT:
- Description: DISTINCT in SELECT statement to retrieve distinct rows.
- Code Example:
```
SELECT DISTINCT *
FROM TableName;
```

Finding and Deleting Duplicates in SQL:

Description: Identify and delete duplicates using a combination of SELECT and DELETE statements.

Code Example:

-- Identify duplicates
SELECT ColumnName, COUNT(*)
FROM TableName
GROUP BY ColumnName
HAVING COUNT(*) > 1;

-- Delete duplicates
DELETE FROM TableName
WHERE ColumnName IN (
    SELECT ColumnName
    FROM TableName
    GROUP BY ColumnName
    HAVING COUNT(*) > 1
);

DISTINCT vs GROUP BY for Identifying Duplicates:
- Description: DISTINCT is used for unique values, while GROUP BY is used for aggregations, including identifying duplicates.
- Code Example (GROUP BY):
```
SELECT ColumnName, COUNT(*)
FROM TableName
GROUP BY ColumnName
HAVING COUNT(*) > 1;
```
SQL DISTINCT with Multiple Columns:
- Description: Retrieves distinct combinations of values from multiple columns.
- Code Example:
```
SELECT DISTINCT Column1, Column2
FROM TableName;
```
Avoiding Pitfalls When Using DISTINCT:
- Description: Be cautious when using DISTINCT, as it might not always work as expected with multiple columns or complex queries.
- Code Example (Pitfall):
```
-- Be cautious with DISTINCT
SELECT DISTINCT Column1, Column2, Column3
FROM TableName;
```

Identifying and Deleting Duplicate Data in SQL:

Description: A comprehensive approach to identify and delete duplicate data using various SQL statements.

Code Example:

-- Identify duplicates
SELECT Column1, Column2, COUNT(*)
FROM TableName
GROUP BY Column1, Column2
HAVING COUNT(*) > 1;

-- Delete duplicates
DELETE FROM TableName
WHERE (Column1, Column2) IN (
    SELECT Column1, Column2
    FROM TableName
    GROUP BY Column1, Column2
    HAVING COUNT(*) > 1
);