info/developer-roadmap
1
0
Fork 0
mirror of https://github.com/kamranahmedse/developer-roadmap.git synced 2025-08-01 06:50:26 +02:00
Code Issues Releases Wiki Activity

Fixing PRIMARY_KEY NULL constraint (#4465)

Browse Source
This commit is contained in:
Akash Sharma
2023-09-12 20:57:19 +05:30
committed by GitHub
parent b565ce9bce
commit 79a656e171
1 changed files with 2 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
src/data/roadmaps/sql/content/105-data-constraints/102-unique.md
View File

@@ -2,7 +2,7 @@
The `UNIQUE` constraint ensures that all values in a column are different; that is, each value in the column should occur only once.
Both the `UNIQUE` and `PRIMARY KEY` constraints provide a guarantee for uniqueness for a column or set of columns. However, a primary key can contain only `NULL`, and each table can have only one primary key. On the other hand, a `UNIQUE` constraint allows for one `NULL` value, and a table can have multiple `UNIQUE` constraints.
Both the `UNIQUE` and `PRIMARY KEY` constraints provide a guarantee for uniqueness for a column or set of columns. However, a primary key cannot contain `NULL` since it uniquely identifies each row, and each table can have only one primary key. On the other hand, a `UNIQUE` constraint allows for one `NULL` value, and a table can have multiple `UNIQUE` constraints.
## Syntax
```sql
@@ -51,4 +51,4 @@ ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
```
Here, `constraint_name` is the name of the `UNIQUE` constraint that you want to drop.
Here, `constraint_name` is the name of the `UNIQUE` constraint that you want to drop.