SQL ADD

What is SQL ADD?

Adds a new column, constraint, index, or partition to an existing database object, most commonly within an ALTER TABLE statement.

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Compatible dialects for SQL ADD: PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, SQLite, Snowflake, Redshift, BigQuery

SQL ADD Full Explanation

ADD is a clause used inside ALTER statements to extend an existing schema object without recreating it. The most frequent use case is ALTER TABLE ... ADD to introduce a new column, but the clause also supports adding constraints (PRIMARY KEY, UNIQUE, CHECK, FOREIGN KEY), indexes, table partitions, and storage parameters depending on the SQL dialect. The operation is normally instantaneous for metadata only, but on large tables some engines need to rewrite or lock the table, which can impact performance or availability. When adding a NOT NULL column without a DEFAULT, many databases will fail because existing rows break the NOT NULL rule. Supplying a DEFAULT lets the engine back-fill existing rows automatically. Some dialects (MySQL, SQL Server) allow positioning keywords such as FIRST or AFTER column_name, while standard SQL does not. ADD applies only to existing objects; use CREATE for new objects and MODIFY/ALTER COLUMN for changing existing columns.

SQL ADD Syntax

-- Add a column
ALTER TABLE table_name
ADD [COLUMN] column_name data_type [column_constraints] [DEFAULT default_value];

-- Add multiple columns (comma-separated)
ALTER TABLE table_name
ADD (
    column1_name data_type,
    column2_name data_type
);

-- Add a table-level constraint
ALTER TABLE table_name
ADD CONSTRAINT constraint_name constraint_type (column_list);

-- Add an index (dialect specific)
ALTER TABLE table_name
ADD INDEX index_name (column_list);

-- Add a partition (dialect specific)
ALTER TABLE table_name
ADD PARTITION partition_definition;

SQL ADD Parameters

table_name (identifier) - Target table to change
column_name (identifier) - Name of the new column
data_type (data type) - Column data type (INT, VARCHAR, etc.)
column_constraints (clause) - Optional NOT NULL, UNIQUE, etc.
default_value (expression) - Value assigned to existing/new rows
constraint_name (identifier) - Name of the new constraint
constraint_type (keyword) - PRIMARY KEY, UNIQUE, CHECK, FOREIGN KEY
column_list (list) - One or more columns separated by commas
partition_definition (clause) - Engine specific partition description

Example Queries Using SQL ADD

-- 1. Add a nullable column
ALTER TABLE customers
ADD signup_source VARCHAR(50);

-- 2. Add a NOT NULL column with default to backfill existing rows
ALTER TABLE orders
ADD order_status VARCHAR(20) DEFAULT 'pending' NOT NULL;

-- 3. Add two columns at once (PostgreSQL, Oracle, SQL Server)
ALTER TABLE products
ADD (
    weight NUMERIC(8,2),
    dimensions JSONB
);

-- 4. Add a table-level UNIQUE constraint
ALTER TABLE employees
ADD CONSTRAINT uq_email UNIQUE (email);

-- 5. MySQL: add an index while altering
ALTER TABLE log_events
ADD INDEX idx_event_ts (event_timestamp);

-- 6. PostgreSQL: add partition (range partitioning)
ALTER TABLE sales_by_month
ADD PARTITION FOR VALUES FROM ('2024-01-01') TO ('2024-02-01');

Expected Output Using SQL ADD

The database catalog is updated
A new column, constraint, index, or partition becomes part of the object schema
Existing applications can immediately reference the new element
If a column was added with a DEFAULT, existing rows receive that value

Use Cases with SQL ADD

Evolve a schema with zero data loss by adding new attributes to a table
Enforce new business rules through additional constraints
Improve query performance by adding indexes during migration scripts
Introduce partitions to manage large historical tables
Apply incremental changes in CI/CD pipelines without downtime

Common Mistakes with SQL ADD

Forgetting to provide a DEFAULT when adding a NOT NULL column, causing the statement to fail
Omitting the COLUMN keyword in dialects that require it (e.g., SQL Server)
Expecting column positioning (FIRST/AFTER) in databases that do not support it
Using ADD to rename or modify existing columns instead of ALTER COLUMN/MODIFY
Attempting to add duplicate constraint names, resulting in errors

First Introduced In

SQL-92 standard (ALTER TABLE ... ADD)

Frequently Asked Questions

How do I add a column to an existing table?

Use ALTER TABLE table_name ADD column_name data_type; Include DEFAULT and NOT NULL if you need to backfill existing rows.

Can I add multiple columns in one command?

PostgreSQL, Oracle, and SQL Server let you add several columns inside one ADD ( ... ) block. MySQL and SQLite require separate ADD clauses per column.

Does adding a NOT NULL column require a default value?

Yes in most databases. Without a DEFAULT, existing rows violate the NOT NULL rule and the statement fails.

How do I add a UNIQUE constraint?

Run ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_list);