Key Concepts

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Column Constraints


Column constraints are the rules applied to the values of individual columns:

  • PRIMARY KEY constraint can be used to uniquely identify the row.
  • UNIQUE columns have a different value for every row.
  • NOT NULL columns must have a value.
  • DEFAULT assigns a default value for the column when no value is specified.

There can be only one PRIMARY KEY column per table and multiple UNIQUE columns.

Lesson 1 of 1
  1. 1
  2. 2
    Nice work! In one line of code, you returned information from a relational database. SELECT * FROM celebs; We’ll take a look at what this code means soon, for now, let’s focus on what relationa…
  3. 3
    The code below is a SQL statement. A statement is text that the database recognizes as a valid command. Statements always end in a semicolon ;. CREATE TABLE table_name ( column_1 data_type, …
  4. 4
    CREATE statements allow us to create a new table in the database. You can use the CREATE statement anytime you want to create a new table from scratch. The statement below creates a new table named…
  5. 5
    The INSERT statement inserts a new row into a table. You can use the INSERT statement when you want to add new records. The statement below enters a record for Justin Bieber into the celebs table. …
  6. 6
    SELECT statements are used to fetch data from a database. In the statement below, SELECT returns all data in the name column of the celebs table. SELECT name FROM celebs; 1. SELECT is a clause …
  7. 7
    The ALTER TABLE statement adds a new column to a table. You can use this command when you want to add columns to a table. The statement below adds a new column twitter_handle to the celebs table. …
  8. 8
    The UPDATE statement edits a row in a table. You can use the UPDATE statement when you want to change existing records. The statement below updates the record with an id value of 4 to have the twit…
  9. 9
    The DELETE FROM statement deletes one or more rows from a table. You can use the statement when you want to delete existing records. The statement below deletes all records in the celeb table with …
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    Congratulations! We’ve learned six commands commonly used to manage data stored in a relational database and how to set constraints on such data. What can we generalize so far? SQL is a programmin…

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