# How to DROP UNIQUE CONSTRAINT in PostgreSQL

> UNIQUE constraints can be dropped by running the ALTER TABLE command with the "DROP CONSTRAINT" clause followed by the constraint’s name.

In PostgreSQL, the “ **DROP CONSTRAINT** ” clause is used with the ALTER TABLE statement to drop any specific constraint from a table. Using the DROP CONSTRAINT clause, users can drop any specific constraint, such as **UNIQUE CONSTRAINT,** FOREIGN KEY CONSTRAINT, CHECK CONSTRAINT, and so on.

PostgreSQL's "DROP CONSTRAINT" clause with UNIQUE constraint is explained in this article. The content that illustrates the demonstration is as follows:

  * How to DROP UNIQUE CONSTRAINT in PostgreSQL?
  * Step 1: Create Table in PostgreSQL
  * Step 2: ADD UNIQUE CONSTRAINT in PostgreSQL
  * Step 3: DROP UNIQUE CONSTRAINT in PostgreSQL



Let's start with the syntax.

 **How to DROP UNIQUE CONSTRAINT in PostgreSQL?**

In the PostgreSQL database, the “ **DROP CONSTRAINT** ” clause removes the rule or policy that is already set using the “ **ADD CONSTRAINT** ” clause. To drop unique constraints from a table, users must follow the syntax stated below:
    
    
    ALTER TABLE tbl_name
    DROP CONSTRAINT constraint_name UNIQUE (col_name);

ALTER TABLE is a command in Postgres used to alter/modify a table, while “ **DROP CONSTRAINT** ” is a clause that drops the existing unique constraint from the table.

 **Step 1: Create Table in PostgreSQL**

First, create a " college " table with the “ **CREATE TABLE** ” statement. Add columns such as **std_id, teach_id, sport_date,** and **notes** in this table:
    
    
    CREATE TABLE college( 
    std_id INTEGER NOT NULL,
    teach_id INTEGER NOT NULL,
    sport_date DATE,
    notes VARCHAR(200)
    );

The “ **CREATE TABLE** ” message verifies that the “ **college** ” table has been successfully created.

 **Step 2: ADD UNIQUE CONSTRAINT in PostgreSQL**

In any existing table, such as “college”, a unique constraint can be added using the “ **ALTER TABLE** ” statement. For this purpose, write the “ **ADD CONSTRAINT** ” clause with a **UNIQUE** constraint and specify the constraint's name, such as “ **running** ”. For this purpose, the statement is as follows:
    
    
    ALTER TABLE college
    ADD CONSTRAINT running UNIQUE   (teach_id);

The **" ALTER TABLE"** message in the output confirms that the unique constraint has been added to the table. After table alteration, the modified table’s structure will be as follows:
    
    
    \d college;

The output shows that the UNIQUE constraint has been added to the selected table.

 **Step 3: DROP UNIQUE CONSTRAINT in PostgreSQL**

The **“ALTER TABLE”** statement is utilized with the **“DROP CONSTRAINT”** clause to drop the constraint. For instance, the statement is given below:
    
    
    ALTER TABLE college
    DROP CONSTRAINT running;

The constraint “ **running** ” in the existing table “ **college** ” has been dropped, which can be verified through the “ **ALTER TABLE”** message.

To verify the working of the **DROP CONSTRAINT** , let’s execute the command mentioned below:
    
    
    \d college;

The output clearly shows that the unique constraint has been successfully dropped from the “college” table.

That's all from this guide.

 **Conclusion**

Execute the **“DROP CONSTRAINT”** with the collaboration of the “ **ALTER TABLE** ” statement to drop a specific constraint from a table. UNIQUE constraints can be dropped by running the **ALTER TABLE** command with the " **DROP CONSTRAINT** " clause followed by the constraint’s name. This blog post presented a step-by-step guide for dropping a unique constraint in Postgres.

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