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A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys.

A key formed by combining at least two or more columns is called composite key.

A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can’t have defaults bound to them.
A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction.
When this clause is used with the DROP command, a parent table can be dropped even when a child table exists.
An outer join includes rows from tables when there are no matching values in the tables.
Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the SELECT statement.
The LIKE keyword allows for string searches. The % and _ sign is used as a wildcard.
The common aggregate and built-in functions are AVG, SUM, MIN, MAX, COUNT and DISTINCT
SUBSTR is used for string manipulation with column name, first position and string length used as arguments.

Eg. SUBSTR(COLUMN_NAME, 1 3) refers to the first three characters in the column COLUMN_NAME.

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