The WHERE clause is used to extract only those records that fulfill a specified criterion.

The WHERE Format :

SELECT FIELD_NAME,FIELD_NAME
FROM TABLE_NAME
WHERE FIELD_NAME OPERATOR_VALUE;

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The CONSTRAINT creating value-exclusive column (no twins) or in other words in a single column UNIQUE constraints can not be two records with the same content.

 

The ADD / DROP CONSTRAINT Format :

ALTER TABLE <TABLE_NAME> 
ADD CONSTRAINT <CONSTRAINT_NAME> UNIQUE <FIELDNAME>;

ALTER TABLE <TABLE_NAME> 
ADD CONSTRAINT <CONSTRAINT_NAME> PRIMARY KEY <FIELDNAME>;

ALTER TABLE <TABLE_NAME> 
DROP CONSTRAINT <CONSTRAINT_NAME>;

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The SQL JOIN Statement linking two or more tables to get all the information field.

The JOIN Format :

SELECT 
    <A.FIELDNAME>, 
    <B.FIELDNAME>,
    ....
FROM <TABLE_A> A
INNER JOIN <TABLE_B> B
ON <A.ID_JOIN>=<B.ID_JOIN>; 

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