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DDL defines a database’s structure; DML changes the data stored in it. In the room analogy, DDL designs the room and its shelves, while DML puts things on the shelves, changes them, or takes them away. The comparison is a teaching aid, not a formal definition of every SQL command.
What DDL and DML mean
DDL stands for data definition language. It covers SQL commands used to define or change database structures, such as tables. DML stands for data manipulation language: commands used to add, change, or remove data in those tables. PostgreSQL presents these topics separately in its documentation: Data Definition and Data Manipulation.
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Think of a table as a room with shelves. DDL lays out the room and its shelves; DML changes what is stored on them. The analogy helps distinguish structure from contents, but database objects and SQL commands are more varied than a room.
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| Question | DDL | DML |
|---|---|---|
| What does it change? | Database structure or definitions | Data stored in tables |
| Typical examples | CREATE, ALTER, DROP |
INSERT, UPDATE, DELETE |
| Room analogy | Design, modify, or remove the room or its shelves | Add, change, or remove the room’s contents |
These examples follow PostgreSQL’s SQL command reference. The labels are useful families, not a complete classification of SQL.
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DDL: define or change database structure
CREATE TABLE defines a table
A table definition establishes a place for data and its structure. For example, CREATE TABLE defines a table in the database.
ALTER TABLE changes a table definition
Use ALTER TABLE to change an existing table’s definition. In the analogy, this is like changing the shelves rather than rearranging the items on them.
DROP TABLE removes a table
DROP TABLE removes the table object. This acts on the structure itself, rather than simply deleting a row from a table.
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INSERT adds rows
INSERT creates rows in a table. PostgreSQL’s insertion documentation demonstrates adding data to a products table.
UPDATE changes rows
UPDATE changes data already held in a table. It does not redefine the table’s structure.
DELETE removes rows
DELETE removes data from a table. It is different from DROP TABLE: the former removes rows, while the latter removes the table object.
Not every SQL command is DDL or DML
DDL and DML explain two important kinds of SQL work, but they do not account for every command. PostgreSQL’s command reference also lists SELECT, transaction controls such as COMMIT and ROLLBACK, and access-control statements. Avoid assuming that every SQL statement fits neatly into one of these two labels.
Whether a command succeeds also depends on context
Transaction mode
In PostgreSQL, transaction mode can restrict commands. Its SET TRANSACTION documentation says a read-only transaction disallows specified data-changing commands, including INSERT, UPDATE, and DELETE, as well as structure-changing commands including CREATE, ALTER, and DROP, subject to documented exceptions. This is PostgreSQL behavior; do not treat it as a universal rule for every database.
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Permissions
A command’s category does not grant permission to run it. PostgreSQL defines distinct privileges, including SELECT, INSERT, UPDATE, DELETE, and CREATE; which privileges apply depends on the database object. See its privileges documentation.
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