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MySQL EXPLAIN and DESCRIBE (Query Analyzer)

Tutorial on the use of EXPLAIN and DESCRIBE commands in MySQL.

EXPLAIN

The EXPLAIN keyword in MySQL provides information about how MySQL executes a SELECT statement. This is a great tool for optimizing your queries, as it shows you how the MySQL query optimizer would execute your query.

Here's a basic example:

EXPLAIN SELECT * FROM employees WHERE salary > 50000;

The EXPLAIN keyword will return a result set with the following columns:

id: The SELECT identifier. This is the sequence in which tables are read.
select_type: The type of SELECT.
table: The table for the output row.
type: The join type.
possible_keys: The possible indexes to choose.
key: The index actually chosen.
key_len: The length of the chosen key.
ref: The columns compared to the index.
rows: The number of rows scanned.
Extra: Additional information.

By analyzing these results, you can understand how your query is being processed and identify potential performance bottlenecks.

DESCRIBE

The DESCRIBE statement provides information about the structure of a table. It's a handy way to know the details of table columns like datatype, whether it's nullable, default values, and other attributes.

Here's an example:

DESCRIBE employees;

The DESCRIBE statement will return a result set with the following columns:

Field: The name of the column.
Type: The type of data the column can hold.
Null: Whether the column can hold NULL values.
Key: Whether the column is indexed.
Default: The default value for the column.
Extra: Any extra attributes and properties for the column.

You can also use DESCRIBE with a LIKE clause to display information for columns with specific patterns:

DESCRIBE employees salary;

This will display information about the salary column in the employees table.

These commands can be very helpful when debugging and optimizing SQL queries, as well as when trying to understand the structure of unfamiliar database tables.

Using EXPLAIN to analyze MySQL queries:
- The EXPLAIN statement is used before a SELECT query to obtain information about how MySQL will execute the query.
```
EXPLAIN SELECT column1, column2 FROM table_name WHERE condition;
```
MySQL query plan analysis with EXPLAIN:
- The output of EXPLAIN provides details about the execution plan, including the order in which tables are accessed, the type of access method used, and possible indexes.
```
EXPLAIN SELECT * FROM orders WHERE customer_id = 1001;
```
DESCRIBE statement in MySQL explained:
- DESCRIBE is an alternative to EXPLAIN and is used to obtain information about the columns of a table.
```
DESCRIBE table_name;
```