# Tables

## What is a Table?

Tables are the core unit of database [schema](https://docs.dolthub.com/concepts/dolt/sql/schema). Tables are defined by a set of columns. Columns can be [primary keys](https://docs.dolthub.com/concepts/dolt/sql/primary-key) which act as a unique identifier for each row. Once a table schema is defined, rows containing data can be inserted into the table.

Table data is stored on disk. The way a database lays out it's table data on disk defines some of the performance characteristics of the database.

## How to use Tables

Structure the data in your database into tables. Define relationships between tables using foreign key [constraints](https://docs.dolthub.com/concepts/dolt/sql/constraints). Use `CREATE` statements to create tables and `ALTER` statements to change their schema.

## Difference between MySQL Table and Dolt Table

A MySQL and Dolt table function the same on the surface. `CREATE` and `ALTER` statements work the same on both.

Dolt and MySQL are [row major](https://en.wikipedia.org/wiki/Row-_and_column-major_order), meaning row values are stored next to each other. However, MySQL stores data in a binary tree structure while Dolt stores table data on disk using a content-addressed binary tree called a [prolly tree](https://docs.dolthub.com/architecture/storage-engine/prolly-tree). This setup makes Dolt [fairly comparable in query performance to MySQL](https://docs.dolthub.com/sql-reference/benchmarks/latency) while also providing history-independence and fast `diff` between versions. Fast `diff` powers Dolt's version control capabilities.

## Interaction with Dolt Version Control

Dolt versions table schema and data. A table in Dolt is akin to a file in Git, it is the unit of change. Tables are the target of `dolt add`.

## Example

```
mysql> show tables;
+----------------+
| Tables_in_docs |
+----------------+
| complex        |
| docs           |
+----------------+
mysql> alter table complex add column c4 blob;
mysql> show create table complex;
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                  |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| complex | CREATE TABLE `complex` (
  `pk1` int NOT NULL,
  `pk2` varchar(47) NOT NULL,
  `c1` tinyint NOT NULL,
  `c2` datetime,
  `c3` json,
  `c4` blob,
  PRIMARY KEY (`pk1`,`pk2`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_bin |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
```


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