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SQL SELECT Statement

What do we use SQL commands for? A common use is to select data from the tables located in a database. Immediately, we see two keywords: we need to SELECT information FROM a table. (Note that a table is a container that resides in the database where the data is stored). Hence we have the most basic SQL structure:

SELECT "column_name" FROM "table_name"

To illustrate the above example, assume that we have the following table:

Table Store_Information

We shall use this table as an example throughout the tutorial (this table will appear in all sections). To select all the stores in this table, we key in,

SELECT store_name FROM Store_Information

Result:

Multiple column names can be selected, as well as multiple table names.

 SQL DISTINCT 

The SELECT keyword allows us to grab all information from a column (or columns) on a table. This, of course, necessarily mean that there will be redundancies. What if we only want to select each DISTINCT element? This is easy to accomplish in SQL. All we need to do is to add DISTINCT after SELECT. The syntax is as follows:

SELECT DISTINCT "column_name"
FROM "table_name"

For example, to select all distinct stores in Table Store_Information,

Table Store_Information

we key in,

SELECT DISTINCT store_name FROM Store_Information

Result:

 SQL WHERE
 

Next, we might want to conditionally select the data from a table. For example, we may want to only retrieve stores with sales above $1,000. To do this, we use the WHERE keyword. The syntax is as follows:

SELECT "column_name"
FROM "table_name"
WHERE "condition"

For example, to select all stores with sales above $1,000 in Table Store_Information,

Table Store_Information

we key in,

SELECT store_name
FROM Store_Information
WHERE Sales > 1000

Result:

SQL AND OR

In the previous section, we have seen that the WHERE keyword can be used to conditionally select data from a table. This condition can be a simple condition (like the one presented in the previous section), or it can be a compound condition. Compound conditions are made up of multiple simple conditions connected by AND or OR. There is no limit to the number of simple conditions that can be present in a single SQL statement.

The syntax for a compound condition is as follows:

SELECT "column_name"
FROM "table_name"
WHERE "simple condition"
{[AND|OR] "simple condition"}+

The {}+ means that the expression inside the bracket will occur one or more times. Note that AND and OR can be used interchangeably. In addition, we may use the parenthesis sign () to indicate the order of the condition.

For example, we may wish to select all stores with sales greater than $1,000 or all stores with sales less than $500 but greater than $275 in Table Store_Information,

Table Store_Information

we key in,

SELECT store_name
FROM Store_Information
WHERE Sales > 1000
OR (Sales < 500 AND Sales > 275)

Result:

 SQL IN

In SQL, there are two uses of the IN keyword, and this section introduces the one that is related to the WHERE clause. When used in this context, we know exactly the value of the returned values we want to see for at least one of the columns. The syntax for using the IN keyword is as follows:

SELECT "column_name"
FROM "table_name"
WHERE "column_name" IN ('value1', 'value2', ...)

The number of values in the parenthesis can be one or more, with each values separated by comma. Values can be numerical or characters. If there is only one value inside the parenthesis, this commend is equivalent to

WHERE "column_name" = 'value1'

For example, we may wish to select all records for the Los Angeles and the San Diego stores in Table Store_Information,

Table Store_Information

we key in,

SELECT *
FROM Store_Information
WHERE store_name IN ('Los Angeles', 'San Diego')

Result: