> For the complete documentation index, see [llms.txt](https://mun.gitbook.io/dbms/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mun.gitbook.io/dbms/sql/mysql-constraints/unique.md).

# UNIQUE

## Unique

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` CHAR(12) NOT NULL,
	`course_code` VARCHAR(40) NOT NULL UNIQUE,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
)
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL UNIQUE,
	`course_code` VARCHAR(7) NOT NULL UNIQUE,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);
SHOW INDEX FROM `course`;
```

## Unique

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE(`course_id`),
	UNIQUE(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE(`course_id`, `course_code`)
);
SHOW INDEX FROM `course`;
```

## Unique Key

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE KEY(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE KEY `uc_course_code`(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE KEY `uc_course_id`(`course_id`),
	UNIQUE KEY `uc_course_code`(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	UNIQUE KEY `uc_course_id_code`
	(`course_id`, `course_code`)
);
SHOW INDEX FROM `course`;
```

## Constraint Unique

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	CONSTRAINT UNIQUE(`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	CONSTRAINT `uc_course_code` UNIQUE (`course_code`)
);
SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	CONSTRAINT `uc_course_id` UNIQUE (`course_id`),
	CONSTRAINT `uc_course_code` UNIQUE (`course_code`)
);
SHOW INDEX FROM `course`;

```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5)),
	CONSTRAINT `uc_course_id_code` 
	UNIQUE (`course_id`, `course_code`)
);
SHOW INDEX FROM `course`;
```

## Alter Table + Unique

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD UNIQUE(`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD UNIQUE(`course_id`);
ALTER TABLE `course` ADD UNIQUE(`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD 
UNIQUE(`course_id`, `course_code`);

SHOW INDEX FROM `course`;
```

## Alter Table + Unique Key

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD 
UNIQUE KEY `uc_course_code` (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD 
UNIQUE KEY `uc_course_id` (`course_id`);
ALTER TABLE `course` ADD 
UNIQUE KEY `uc_course_code` (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD UNIQUE KEY 
`uc_course_id_code` (`course_id`, `course_code`);

SHOW INDEX FROM `course`;
```

## Alter Table + Constraint

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD 
CONSTRAINT UNIQUE (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD CONSTRAINT 
`uc_course_code` UNIQUE (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD CONSTRAINT 
`uc_course_id` UNIQUE (`course_id`);
ALTER TABLE `course` ADD CONSTRAINT 
`uc_course_code` UNIQUE (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

ALTER TABLE `course` ADD CONSTRAINT `uc_course_id_code` 
UNIQUE (`course_id`, `course_code`);

SHOW INDEX FROM `course`;
```

## Create Unique Index

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

CREATE UNIQUE INDEX `uc_course_code` 
ON `course` (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

CREATE UNIQUE INDEX `uc_course_id` 
ON `course` (`course_id`);
CREATE UNIQUE INDEX `uc_course_code` 
ON `course` (`course_code`);

SHOW INDEX FROM `course`;
```

```sql
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

CREATE UNIQUE INDEX `uc_course_id_code` 
ON `course` (`course_id`, `course_code`);

SHOW INDEX FROM `course`;
```

## Drop Index

```sql
DROP TABLE IF EXISTS `course`;
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL UNIQUE,
	`course_code` VARCHAR(7) NOT NULL UNIQUE,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

SHOW INDEX FROM `course`;
DROP INDEX `course_id` ON `course`;
SHOW INDEX FROM `course`;
DROP INDEX `course_code` ON `course`;
SHOW INDEX FROM `course`;
```

```sql
DROP TABLE IF EXISTS `course`;
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

CREATE UNIQUE INDEX `uc_course_id_code` 
ON `course` (`course_id`, `course_code`);

SHOW INDEX FROM `course`;
DROP INDEX `uc_course_id_code` ON `course`;
SHOW INDEX FROM `course`;
```

## Drop Index + Alter Table

```sql
DROP TABLE IF EXISTS `course`;
CREATE TABLE IF NOT EXISTS `course`(
	`course_id` INT(11) NOT NULL,
	`course_code` VARCHAR(7) NOT NULL,
	`course_title` VARCHAR(40) NOT NULL,
	`course_dept` VARCHAR(4) NOT NULL,
	`course_credit` FLOAT(3,1) 
	CHECK (`course_credit` IN (3.0, 1.5))
);

CREATE UNIQUE INDEX `uc_course_id` 
ON `course` (`course_id`);
CREATE UNIQUE INDEX `uc_course_code` 
ON `course` (`course_code`);

SHOW INDEX FROM `course`;
ALTER TABLE `course` DROP INDEX `uc_course_id`;
SHOW INDEX FROM `course`;
ALTER TABLE `course` DROP INDEX `uc_course_code`;
SHOW INDEX FROM `course`;
```


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