What is the difference between constraint and foreign key?

Question

SQL

I can set the integrity constraint both through constraint and through foreign key. Why is one better than the other?

Answer 1

You are confusing sweet with soft.
Constraint constraint - when you set table parameters with create/alter table
you are saying that a constraint is being set for such and such a field.
Constraints can be primary key, foreign key, unique, default, check, etc. - these are all constraints.
Here is the table:

CREATE TABLE MYTABLE  (
   ID BIGINT NOT NULL CONSTRAINT PK_MYTABLE PRIMARY KEY,
   CLIENT BIGINT NOT NULL CONSTRAINT FK_MYTABLE_CLIENT FOREIGN KEY REFERENCES CLIENTS(ID),
   EMAIL NVARCHAR(100) NOT NULL CONSTRAINT UQ_MYTABLE_EMAIL UNIQUE,
   ACTIVE BIT NOT NULL CONSTRAINT DF_MYTABLE_ACTIVE DEFAULT (1)
)

Which translates as:
Create a table MYTABLE (with fields):
ID integer 64 bits, do not allow empty, constraint named PK_MYTABLE of the form "Primary key",
CLIENT integer 64 bits, do not allow empty, constraint named FK_MYTABLE_CLIENT of the form "Foreign key", which refers to the CLIENTS table, field ID,
EMAIL string of length 100, do not allow empty, constraint named UQ_MYTABLE_EMAIL of the form "Must be unique",
ACTIVE bit, do not allow empty, constraint named DF_MYTABLE_ACTIVE of the form "Default value" equal to 1
Yes and alternative syntax where the constraint keyword is not used.
It is mainly used when you need to explicitly set a name for a constraint.

Answer 2

foreign key

Link to another table.

constraint

Limitation. May not be related to another table. For example, a date is limited to a certain period.