SQL-Data StatementsSQL-Data Statements perform query and modification on database tables and columns. This subset of SQL is also called the Data Manipulation Language for SQL (SQL DML).
SELECT Statement BasicsThe SQL SELECT statement queries data from tables in the database. The statement begins with the SELECT keyword. The basic SELECT statement has 3 clauses:
The FROM clause specifies the tables accessed.
The WHERE clause specifies which table rows are used. The WHERE clause is optional; if missing, all table rows are used.
SELECT name FROM s WHERE city='Rome'This query accesses rows from the table - s. It then filters those rows where the city column contains Rome. Finally, the query retrieves the name column from each filtered row. Using the example s table, this query produces:
- The FROM clause accesses the s table. Contents:
sno name city S1 Pierre Paris S2 John London S3 Mario Rome
- The WHERE clause filters the rows of the FROM table to use those whose city column contains Rome. This chooses a single row from s:
sno name city S3 Mario Rome
- The SELECT clause retrieves the name column from the rows filtered by the WHERE clause:
- SELECT Clause -- specifies the table columns retrieved
- FROM Clause -- specifies the tables to be accessed
- WHERE Clause -- specifies which rows in the FROM tables to use
SELECT [ALL|DISTINCT] select-listselect-list is a list of column names separated by commas. The ALL and DISTINCT specifiers are optional. DISTINCT specifies that duplicate rows are discarded. A duplicate row is when each corresponding select-list column has the same value. The default is ALL, which retains duplicate rows.
SELECT descr, color FROM pThe column names in the select list can be qualified by the appropriate table name:
SELECT p.descr, p.color FROM pA column in the select list can be renamed by following the column name with the new name. For example:
SELECT name supplier, city location FROM sThis produces:
A special select list consisting of a single '*' requests all columns in all tables in the FROM clause. For example,
SELECT sp.* FROM sp
An unqualified * cannot be combined with other elements in the select list; it must be stand alone. However, a qualified * can be combined with other elements. For example,
SELECT sp.*, city
FROM sp, s
FROM clause always follows the SELECT clause. It lists the tables accessed by the query. For example,
SELECT * FROM sWhen the From List contains multiple tables, commas separate the table names. For example,
SELECT sp.*, cityWhen the From List has multiple tables, they must be joined together.
FROM sp, s
SELECT supplier.name FROM s supplierThe new name is known as the correlation (or range) name for the table.
WHERE clause is optional. When specified, it always follows the FROM clause. The WHERE clause filters rows from the FROM clause tables. Omitting the WHERE clause specifies that all rows are used.
Following the WHERE keyword is a logical expression, also known as a predicate.
The predicate evaluates to a SQL logical value -- true, false or unknown. The most basic predicate is a comparison:
color = 'Red'This predicate returns:
- true -- if the color column contains the string value -- 'Red',
- false -- if the color column contains another string value (not 'Red'), or
- unknown -- if the color column contains null.
The = (equals) comparison operator compares two values for equality. Additional comparison operators are:
- > -- greater than
- < -- less than
- >= -- greater than or equal to
- <= -- less than or equal to
- <> -- not equal to
SELECT * FROM sp WHERE qty >= 200
Both operands of a comparison should be the same data type, however automatic conversions are performed between numeric, datetime and interval types. The CAST expression provides explicit type conversions;
Extended ComparisonsIn addition to the basic comparisons described above, SQL supports extended comparison operators -- BETWEEN, IN, LIKE and IS NULL.
- BETWEEN Operator
The BETWEEN operator implements a range comparison, that is, it tests whether a value is between two other values. BETWEEN comparisons have the following format:
value-1 [NOT] BETWEEN value-2 AND value-3
value-1 >= value-2 AND value-1 <= value-3Or, if NOT is included:
NOT (value-1 >= value-2 AND value-1 <= value-3)For example,
WHERE qty BETWEEN 50 and 500
The IN operator implements comparison to a list of values, that is, it tests whether a value matches any value in a list of values. IN comparisons have the following general format:
value-1 [NOT] IN ( value-2 [, value-3] ... )This comparison tests if value-1 matches value-2 or matches value-3, and so on. It is equivalent to the following logical predicate:
value-1 = value-2 [ OR value-1 = value-3 ] ...or if NOT is included:
NOT (value-1 = value-2 [ OR value-1 = value-3 ] ...)For example,
SELECT name FROM s WHERE city IN ('Rome','Paris')
The LIKE operator implements a pattern match comparison, that is, it matches a string value against a pattern string containing wild-card characters.
The wild-card characters for LIKE are percent -- '%' and underscore -- '_'. Underscore matches any single character. Percent matches zero or more characters.
LIKE comparison has the following general format:
value-1 [NOT] LIKE value-2 [ESCAPE value-3]All values must be string (character). This comparison uses value-2 as a pattern to match value-1. The optional ESCAPE sub-clause specifies an escape character for the pattern, allowing the pattern to use '%' and '_' (and the escape character) for matching. The ESCAPE value must be a single character string. In the pattern, the ESCAPE character precedes any character to be escaped.
For example, to match a string ending with '%', use:
x LIKE '%/%' ESCAPE '/'A more contrived example that escapes the escape character:
y LIKE '/%//%' ESCAPE '/'... matches any string beginning with '%/'.
The optional NOT reverses the result so that:
z NOT LIKE 'abc%'is equivalent to:
NOT z LIKE 'abc%'
A database null in a table column has a special meaning -- the value of the column is not currently known (missing), however its value may be known at a later time. A database null may represent any value in the future, but the value is not available at this time. Since two null columns may eventually be assigned different values, one null can't be compared to another in the conventional way. The following syntax is illegal in SQL:
WHERE qty = NULLA special comparison operator -- IS NULL, tests a column for null. It has the following general format:
value-1 IS [NOT] NULLThis comparison returns true if value-1 contains a null and false otherwise. The optional NOT reverses the result:
value-1 IS NOT NULLis equivalent to:
NOT value-1 IS NULLFor example,
SELECT * FROM sp WHERE qty IS NULL
Logical OperatorsThe logical operators are AND, OR, NOT. They take logical expressions as operands and produce a logical result (True, False, Unknown). In logical expressions, parentheses are used for grouping.
- AND Operator
The AND operator combines two logical operands. The operands are comparisons or logical expressions. It has the following general format:
predicate-1 AND predicate-2
- True -- if both operands evaluate to true
- False -- if either operand evaluates to false
- Unknown -- otherwise (one operand is true and the other is unknown or both are unknown)
Truth tables for AND:
WHERE sno='S3' AND qty <>
The OR operator combines two logical operands. The operands are comparisons or logical expressions. It has the following general format:
predicate-1 OR predicate-2OR returns:
- True -- if either operand evaluates to true
- False -- if both operands evaluate to false
- Unknown -- otherwise (one operand is false and the other is unknown or both are unknown)
Truth tables for OR:
WHERE sno='S3' OR city = 'London'
AND has a higher precedence than OR, so the following expression:
a OR b AND cis equivalent to:
a OR (b AND c)
The NOT operator inverts the result of a comparison expression or a logical expression. It has the following general format:
Truth tables for NOT:
WHERE NOT sno = 'S3'