SQL Data Definition and Data Types
SQL uses the terms table, row, and column for the formal relational model terms relation, tuple, and attribute, respectively. We will use the corresponding terms inter-changeably. The main SQL command for data definition is the CREATE statement, which can be used to create schemas, tables (relations), and domains (as well as other constructs such as views, assertions, and triggers). Before we describe the rel-evant CREATE statements, we discuss schema and catalog concepts in Section 4.1.1 to place our discussion in perspective. Section 4.1.2 describes how tables are created, and Section 4.1.3 describes the most important data types available for attribute specification. Because the SQL specification is very large, we give a description of the most important features. Further details can be found in the various SQL stan-dards documents (see end-of-chapter bibliographic notes).
1. Schema and Catalog Concepts in SQL
Early versions of SQL did not include the concept of a relational database schema; all tables (relations) were considered part of the same schema. The concept of an SQL schema was incorporated starting with SQL2 in order to group together tables and other constructs that belong to the same database application. An SQL schema is identified by a schema name, and includes an authorization identifier to indicate the user or account who owns the schema, as well as descriptors for each element in the schema. Schema elements include tables, constraints, views, domains, and other constructs (such as authorization grants) that describe the schema. A schema is created via the CREATE SCHEMA statement, which can include all the schema elements’ definitions. Alternatively, the schema can be assigned a name and authorization identifier, and the elements can be defined later. For example, the following statement creates a schema called COMPANY, owned by the user with authorization iden-tifier ‘Jsmith’. Note that each statement in SQL ends with a semicolon.
CREATE SCHEMA COMPANY AUTHORIZATION ‘Jsmith’;
In general, not all users are authorized to create schemas and schema elements. The privilege to create schemas, tables, and other constructs must be explicitly granted to the relevant user accounts by the system administrator or DBA.
In addition to the concept of a schema, SQL uses the concept of a catalog—a named collection of schemas in an SQL environment. An SQL environment is basically an installation of an SQL-compliant RDBMS on a computer system. A catalog always contains a special schema called INFORMATION_SCHEMA, which provides information on all the schemas in the catalog and all the element descriptors in these schemas. Integrity constraints such as referential integrity can be defined between relations only if they exist in schemas within the same catalog. Schemas within the same catalog can also share certain elements, such as domain definitions.
2. The CREATE TABLE Command in SQL
The CREATE TABLE command is used to specify a new relation by giving it a name and specifying its attributes and initial constraints. The attributes are specified first, and each attribute is given a name, a data type to specify its domain of values, and any attribute constraints, such as NOT NULL. The key, entity integrity, and referential integrity constraints can be specified within the CREATE TABLE statement after the attributes are declared, or they can be added later using the ALTER TABLE command (see Chapter 5). Figure 4.1 shows sample data definition statements in SQL for the COMPANY relational database schema shown in Figure 3.7.
Typically, the SQL schema in which the relations are declared is implicitly specified in the environment in which the CREATE TABLE statements are executed. Alternatively, we can explicitly attach the schema name to the relation name, sepa-rated by a period. For example, by writing
CREATE TABLE COMPANY.EMPLOYEE ...
CREATE TABLE EMPLOYEE ...
as in Figure 4.1, we can explicitly (rather than implicitly) make the EMPLOYEE table part of the COMPANY schema.
The relations declared through CREATE TABLE statements are called base tables (or base relations); this means that the relation and its tuples are actually created and stored as a file by the DBMS. Base relations are distinguished from virtual relations, created through the CREATE VIEW statement (see Chapter 5), which may or may not correspond to an actual physical file. In SQL, the attributes in a base table are considered to be ordered in the sequence in which they are specified in the CREATE TABLE statement. However, rows (tuples) are not considered to be ordered within a relation.
It is important to note that in Figure 4.1, there are some foreign keys that may cause errors because they are specified either via circular references or because they refer to a table that has not yet been created. For example, the foreign key Super_ssn in the EMPLOYEE table is a circular reference because it refers to the table itself. The foreign key Dno in the EMPLOYEE table refers to the DEPARTMENT table, which has
not been created yet. To deal with this type of problem, these constraints can be left out of the initial CREATE TABLE statement, and then added later using the ALTER TABLE statement (see Chapter 5). We displayed all the foreign keys in Figure 4.1 to show the complete COMPANY schema in one place.
3. Attribute Data Types and Domains in SQL
The basic data types available for attributes include numeric, character string, bit string, Boolean, date, and time.
Numeric data types include integer numbers of various sizes (INTEGER or INT, and SMALLINT) and floating-point (real) numbers of various precision (FLOAT or REAL, and DOUBLE PRECISION). Formatted numbers can be declared by using DECIMAL(i,j)—or DEC(i,j) or NUMERIC(i,j)—where i, the precision, is the total number of decimal digits and j, the scale, is the number of digits after the decimal point. The default for scale is zero, and the default for precision is implementation-defined.
Character-string data types are either fixed length—CHAR(n) or CHARACTER(n), where n is the number of characters—or varying length— VARCHAR(n) or CHAR VARYING(n) or CHARACTER VARYING(n), where n is the maximum number of characters. When specifying a literal string value, it is placed between single quotation marks (apostrophes), and it is case sensi-tive (a distinction is made between uppercase and lowercase). For fixed-length strings, a shorter string is padded with blank characters to the right. For example, if the value ‘Smith’ is for an attribute of type CHAR(10), it is padded with five blank characters to become ‘Smith ’ if needed. Padded blanks are generally ignored when strings are compared. For comparison purposes, strings are considered ordered in alphabetic (or lexicographic) order; if a string str1 appears before another string str2 in alphabetic order, then str1 is considered to be less than str2. There is also a concatenation operator denoted by || (double vertical bar) that can concatenate two strings in SQL. For example, ‘abc’ || ‘XYZ’ results in a single string ‘abcXYZ’. Another variable-length string data type called CHARACTER LARGE OBJECT or CLOB is also available to specify columns that have large text values, such as documents. The CLOB maximum length can be specified in kilobytes (K), megabytes (M), or gigabytes (G). For example, CLOB(20M) specifies a max-imum length of 20 megabytes.
Bit-string data types are either of fixed length n—BIT(n)—or varying length—BIT VARYING(n), where n is the maximum number of bits. The default for n, the length of a character string or bit string, is 1. Literal bit strings are placed between single quotes but preceded by a B to distinguish them from character strings; for example, B‘10101’. Another variable-length bitstring data type called BINARY LARGE OBJECT or BLOB is also available to specify columns that have large binary values, such as images. As for CLOB, the maximum length of a BLOB can be specified in kilobits (K), megabits (M), or gigabits (G). For example, BLOB(30G) specifies a maxi-mum length of 30 gigabits.
A Boolean data type has the traditional values of TRUE or FALSE. In SQL, because of the presence of NULL values, a three-valued logic is used, so a third possible value for a Boolean data type is UNKNOWN. We discuss the need for UNKNOWN and the three-valued logic in Chapter 5.
The DATE data type has ten positions, and its components are YEAR, MONTH, and DAY in the form YYYY-MM-DD. The TIME data type has at least eight positions, with the components HOUR, MINUTE, and SECOND in the form HH:MM:SS. Only valid dates and times should be allowed by the SQL implementation. This implies that months should be between 1 and 12 and dates must be between 1 and 31; furthermore, a date should be a valid date for the corresponding month. The < (less than) comparison can be used with dates or times—an earlier date is considered to be smaller than a later date, and similarly with time. Literal values are represented by single-quoted strings preceded by the keyword DATE or TIME; for example, DATE ‘2008-09-27’ or TIME ‘09:12:47’. In addition, a data type TIME(i), where i is called time fractional seconds precision, specifies i + 1 additional positions for TIME—one position for an additional period (.) separator character, and i positions for specifying decimal fractions of a second. A TIME WITH TIME ZONE data type includes an additional six positions for specifying the displacement from the standard universal time zone, which is in the range +13:00 to –12:59 in units of HOURS:MINUTES. If WITH TIME ZONE is not included, the default is the local time zone for the SQL session.
Some additional data types are discussed below. The list of types discussed here is not exhaustive; different implementations have added more data types to SQL.
A timestamp data type (TIMESTAMP) includes the DATE and TIME fields, plus a minimum of six positions for decimal fractions of seconds and an optional WITH TIME ZONE qualifier. Literal values are represented by single-quoted strings preceded by the keyword TIMESTAMP, with a blank space between data and time; for example, TIMESTAMP ‘2008-09-27 09:12:47.648302’.
Another data type related to DATE, TIME, and TIMESTAMP is the INTERVAL data type. This specifies an interval—a relative value that can be used to increment or decrement an absolute value of a date, time, or timestamp. Intervals are qualified to be either YEAR/MONTH intervals or DAY/TIME intervals.
The format of DATE, TIME, and TIMESTAMP can be considered as a special type of string. Hence, they can generally be used in string comparisons by being cast (or coerced or converted) into the equivalent strings.
It is possible to specify the data type of each attribute directly, as in Figure 4.1; alter-natively, a domain can be declared, and the domain name used with the attribute specification. This makes it easier to change the data type for a domain that is used by numerous attributes in a schema, and improves schema readability. For example, we can create a domain SSN_TYPE by the following statement:
CREATE DOMAIN SSN_TYPE AS CHAR(9);
We can use SSN_TYPE in place of CHAR(9) in Figure 4.1 for the attributes Ssn and
Super_ssn of EMPLOYEE, Mgr_ssn of DEPARTMENT, Essn of WORKS_ON, and Essn of DEPENDENT. A domain can also have an optional default specification via a DEFAULT clause, as we discuss later for attributes. Notice that domains may not be available in some implementations of SQL.
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