SQL COLUMN

What is a SQL COLUMN?

COLUMN defines or references a field within a table and appears mainly in DDL statements such as CREATE TABLE or ALTER TABLE.

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Compatible dialects for SQL COLUMN: PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, SQLite, DB2, Snowflake, Redshift—virtually all relational databases support the COLUMN keyword within DDL.

SQL COLUMN Full Explanation

In SQL, COLUMN is not usually executed as a standalone command but is a reserved keyword that identifies a single field of structured data stored in a table. It is crucial in Data Definition Language (DDL) operations that create, modify, or remove table structure. Typical contexts include CREATE TABLE, where each column is described by a name and data type, and ALTER TABLE, where columns are added, dropped, renamed, or modified. A column stores one data value per row, has an associated data type, may allow or disallow NULLs, and can carry constraints such as PRIMARY KEY, UNIQUE, CHECK, or DEFAULT. Some dialects (Oracle, SQL*Plus) expose a client-side COLUMN command that formats query output, but that usage is tool-specific and not part of ANSI SQL. Important caveats: renaming or dropping a column can invalidate views, triggers, or application code; altering data type may require casting or table locks; and most RDBMSs disallow dropping a column that is referenced by a constraint unless the constraint is removed first.

SQL COLUMN Syntax

-- Creating columns
a) CREATE TABLE table_name (
       column_name data_type [column_constraints],
       ...
   );

-- Adding a column later
b) ALTER TABLE table_name ADD COLUMN column_name data_type [column_constraints];

-- Modifying a column (dialect-specific)
c) ALTER TABLE table_name ALTER COLUMN column_name SET DATA TYPE new_data_type;

-- Dropping a column
d) ALTER TABLE table_name DROP COLUMN column_name;

SQL COLUMN Parameters

column_name (identifier) - Name of the column to create or alter
data_type (type) - SQL data type of the column (INTEGER, VARCHAR, etc.)
column_constraints (clause) - Optional constraints like NOT NULL, DEFAULT, UNIQUE, CHECK

Example Queries Using SQL COLUMN

-- 1. Create table with multiple columns
CREATE TABLE employees (
    id SERIAL PRIMARY KEY,
    first_name VARCHAR(50) NOT NULL,
    last_name  VARCHAR(50) NOT NULL,
    hired_at   DATE DEFAULT CURRENT_DATE
);

-- 2. Add a new column for email
ALTER TABLE employees
ADD COLUMN email VARCHAR(255) UNIQUE;

-- 3. Change data type of hired_at to TIMESTAMP
ALTER TABLE employees
ALTER COLUMN hired_at TYPE TIMESTAMP;

-- 4. Drop the email column
ALTER TABLE employees
DROP COLUMN email;

Expected Output Using SQL COLUMN

The employees table is created with four columns.
A new email column appears; existing rows have NULL values.
hired_at now stores date and time; data is cast where possible.
The email column and its data are removed from the table.

Use Cases with SQL COLUMN

Designing a brand-new table schema.
Adding new data fields to an existing application without recreating the table.
Removing obsolete fields to keep schema lean.
Changing data type or constraints after business requirements evolve.

Common Mistakes with SQL COLUMN

Forgetting to specify a data type when adding a column.
Dropping a column still referenced by a view or constraint.
Using COLUMN as a standalone command in engines that do not support it.
Renaming a column without updating application code.
Omitting LOCK or downtime considerations when altering large tables.

First Introduced In

SQL-92 standard (ALTER TABLE ADD/DROP COLUMN)

Frequently Asked Questions

What does a column represent in a relational table?

A column represents a single attribute of the entity modeled by the table. Each row stores one value for that attribute.

Do I need to specify NOT NULL when creating a column?

If data must always be present, add NOT NULL. Otherwise the column accepts NULLs by default, which may allow incomplete data.

How do I change a column's data type safely?

Create a new column with the desired type, backfill data with CAST, update code, then drop the old column. This avoids risky in-place conversions on large tables.

What happens to indexes when I drop a column?

Indexes that include the column are automatically dropped. Check index definitions first to avoid unexpected performance changes.