6.6: Introduction to Structure Query Language (SQL)

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SQL is the universal query language of relational database management systems (DBMS) that is almost always behind user-friendly GUIs. In this section, we will briefly talk about SQL queries. We will visit SQL in more detail in later chapters.

SQL statements can also include a SQL comment, which is a block of text used to document the SQL statement but not executed as part of the statement. SQL comments are enclosed in the symbols /* and */, and any text between these symbols is ignored when the SQL statement is executed. Here is an example:

/* SQL-Query-Ch02 */

The fundamental statement of SQL query can apply to Microsoft Access, SQL Server, Oracle Database, and MySQL. The basic form of SQL queries uses SQL SELECT – FROM – WHERE framework. Here are some basic specifications:

The SQL SELECT statement specifies which columns are to be listed in the query results.
The SQL FROM statement specifies which tables are to be listed in the query results.
The SQL WHERE statement specifies which rows are to be listed in the query results.

Reading Specified Columns from Single Table – to obtain values from the SKU_Data table, we write a SQL SELECT statement that contains all of the column names in the table:

/* SQL-Query-Ch_02 */

Below is a SQL queries from a single table, which obtains just the value of the Department and Buyer columns of the SKU_Data table:

The Catalog_SKU_2019 table shows DateOnWebSite. To see these items, we use the follow query: /* SQL-Query-Ch_02 */