Page 162 - SQL

, we are not returning any data from B. Thus a join is unnecessary, and the engine knows no
values from B are to be returned, thus no performance hit for using *. Similarly COUNT(*) is fine as it
also doesn't actually return any of the columns, so only needs to read and process those that are
used for filtering purposes.

Selecting with Condition

The basic syntax of SELECT with WHERE clause is:

SELECT column1, column2, columnN
FROM table_name
WHERE [condition]

The [condition] can be any SQL expression, specified using comparison or logical operators like >,
<, =, <>, >=, <=, LIKE, NOT, IN, BETWEEN etc.

The following statement returns all columns from the table 'Cars' where the status column is
'READY':

SELECT * FROM Cars WHERE status = 'READY'

See WHERE and HAVING for more examples.

Select Individual Columns

SELECT
PhoneNumber,
Email,
PreferredContact
FROM Customers

This statement will return the columns PhoneNumber, Email, and PreferredContact from all rows of the
Customers table. Also the columns will be returned in the sequence in which they appear in the
SELECT clause.

The result will be:

PhoneNumber Email PreferredContact

3347927472 william.jones@example.com PHONE

2137921892 dmiller@example.net EMAIL

NULL richard0123@example.com EMAIL

If multiple tables are joined together, you can select columns from specific tables by specifying the
table name before the column name: [table_name].[column_name]

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