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15.5: Structured Query Language summary

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    3258
  • [ "article:topic", "authorname:severancec", "python (language)", "license:ccbyncsa" ]

    So far, we have been using the Structured Query Language in our Python examples and have covered many of the basics of the SQL commands. In this section, we look at the SQL language in particular and give an overview of SQL syntax.

    Since there are so many different database vendors, the Structured Query Language (SQL) was standardized so we could communicate in a portable manner to database systems from multiple vendors.

    A relational database is made up of tables, rows, and columns. The columns generally have a type such as text, numeric, or date data. When we create a table, we indicate the names and types of the columns:

    CREATE TABLE Tracks (title TEXT, plays INTEGER)

    To insert a row into a table, we use the SQL INSERT command:

    INSERT INTO Tracks (title, plays) VALUES ('My Way', 15)

    The INSERT statement specifies the table name, then a list of the fields/columns that you would like to set in the new row, and then the keyword VALUES and a list of corresponding values for each of the fields.

    The SQL SELECT command is used to retrieve rows and columns from a database. The SELECT statement lets you specify which columns you would like to retrieve as well as a WHEREclause to select which rows you would like to see. It also allows an optional ORDER BY clause to control the sorting of the returned rows.

    SELECT * FROM Tracks WHERE title = 'My Way'

    Using * indicates that you want the database to return all of the columns for each row that matches the WHERE clause.

    Note, unlike in Python, in a SQL WHERE clause we use a single equal sign to indicate a test for equality rather than a double equal sign. Other logical operations allowed in a WHEREclause include <><=>=!=, as well as AND and OR and parentheses to build your logical expressions.

    You can request that the returned rows be sorted by one of the fields as follows:

    SELECT title,plays FROM Tracks ORDER BY title

    To remove a row, you need a WHERE clause on an SQL DELETE statement. The WHERE clause determines which rows are to be deleted:

    DELETE FROM Tracks WHERE title = 'My Way'

    It is possible to UPDATE a column or columns within one or more rows in a table using the SQL UPDATE statement as follows:

    UPDATE Tracks SET plays = 16 WHERE title = 'My Way'

    The UPDATE statement specifies a table and then a list of fields and values to change after the SET keyword and then an optional WHERE clause to select the rows that are to be updated. A single UPDATE statement will change all of the rows that match the WHERE clause. If a WHERE clause is not specified, it performs the UPDATE on all of the rows in the table.

    These four basic SQL commands (INSERT, SELECT, UPDATE, and DELETE) allow the four basic operations needed to create and maintain data.