free barcode generator using vb.net Figure 6-1: SELECT statement syntax diagram in Software

Drawing QR Code in Software Figure 6-1: SELECT statement syntax diagram

Figure 6-1: SELECT statement syntax diagram
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The SELECT clause lists the data items to be retrieved by the SELECT statement. The
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items may be columns from the database, or columns to be calculated by SQL as it performs the query. The SELECT clause is described in later sections of this chapter. The FROM clause lists the tables that contain the data to be retrieved by the query. Queries that draw their data from a single table are described in this chapter. More complex queries that combine data from two or more tables are discussed in 7. The WHERE clause tells SQL to include only certain rows of data in the query results. A search condition is used to specify the desired rows. The basic uses of the WHERE clause are described later in this chapter. Those that involve subqueries are discussed in 9. The GROUP BY clause specifies a summary query. Instead of producing one row of query results for each row of data in the database, a summary query groups together similar rows and then produces one summary row of query results for each group. Summary queries are described in 8. The HAVING clause tells SQL to include only certain groups produced by the GROUP BY clause in the query results. Like the WHERE clause, it uses a search condition to specify the desired groups. The HAVING clause is described in 8. The ORDER BY clause sorts the query results based on the data in one or more columns. If it is omitted, the query results are not sorted. The ORDER BY clause is described later in this chapter.
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The SELECT Clause
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The SELECT clause that begins each SELECT statement specifies the data items to be retrieved by the query. The items are usually specified by a select list, a list of select items separated by commas. Each select item in the list generates a single column of query results, in left-to-right order. A select item can be: a column name, identifying a column from the table(s) named in the FROM clause. When a column name appears as a select item, SQL simply takes the value of that column from each row of the database table and places it in the corresponding row of query results. a constant, specifying that the same constant value is to appear in every row of the query results. a SQL expression, indicating that SQL must calculate the value to be placed into the query results, in the style specified by the expression. Each type of select item is described later in this chapter.
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The FROM Clause
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The FROM clause consists of the keyword FROM, followed by a list of table specifications separated by commas. Each table specification identifies a table containing data to be retrieved by the query. These tables are called the source tables of the query (and of the SELECT statement) because they are the source of all of the data in the query results. All of the queries in this chapter have a single source table, and every FROM clause contains a single table name.
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Query Results
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The result of a SQL query is always a table of data, just like the tables in the database. If you type a SELECT statement using interactive SQL, the DBMS displays the query results in tabular form on your computer screen. If a program sends a query to the DBMS using programmatic SQL, the table of query results is returned to the program. In either case, the query results always have the same tabular, row/column format as the actual tables in the database, as shown in Figure 6-2. Usually the query results will be a table with several columns and several rows. For example, this query produces a table of three columns (because it asks for three items of data) and ten rows (because there are ten salespeople):
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