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The current system date in the database The current date at the client application side The system date and exact time, with time zone information The system date and exact time, with time zone information, at the client application side The name used to connect to the database

The difference between dates (and timestamps) at the database side and those at the client application side can be relevant if you are connected over a network connection with a database in a remote location. Users commonly make mistakes by forgetting to include quotes in SQL statements. Consider the following SQL statement fragment: ...WHERE LOCATION = UTRECHT... LOCATION and UTRECHT are both interpreted by Oracle as variable names (column names), although the following was probably the real intention: ...WHERE LOCATION = 'UTRECHT'...

An operator does something. Operands are the victims of operations; that is, operands serve as input for operators. Sometimes, operators need only a single operand (in which case, they are also referred to as monadic operators), but most operators need two or more operands.

The SQL operators are divided in four categories, where the differentiating factor is the operand datatype: Arithmetic operators Alphanumeric operators Comparison operators Logical operators

The SQL language supports four arithmetic operators, as shown in Table 2-5. Table 2-5. SQL Arithmetic Operators

+ * /

You can apply arithmetic operators only on NUMBER values; however, there are some exceptions: If you subtract two DATE values, you get the difference between those two dates, expressed in days. You can add a DATE and an INTERVAL value, which results in another date. If you add a DATE and a NUMBER, the number is interpreted as an interval expressed in days.

SQL offers only one alphanumeric operator, allowing you to concatenate string expressions: ||. This modest number of operators is compensated for by the overwhelming number of alphanumeric functions in SQL, which are discussed in 5. For an example of the use of the concatenation operator, see Table 2-8, later in this chapter.

The comparison operators allow you to formulate conditions in SQL. Table 2-6 shows the comparison operators available in SQL.

< > = <= >= <> or !=

Expressions with comparison operators are also referred to as predicates or Boolean expressions. These expressions evaluate to TRUE or FALSE. Sometimes, the outcome is UNKNOWN, such as when you have rows with missing information. We will revisit this topic in more detail in 4, when we discuss null values.

SQL also offers three operators whose operands are conditions: the logical (or Boolean) operators. Table 2-7 lists these operators. Table 2-7. SQL Logical Operators

An expression is a well-formed string containing variables, constants, operators, or functions. Just like constants, expressions always have a certain datatype. See Table 2-8 for some examples of expressions.

3 + 4 ENAME || ', ' || INIT LOCATION = 'Utrecht' 12*MSAL > 20000 AND COMM >= 100 BDATE + INTERVAL '16' YEAR 999

The last example in Table 2-8 shows that the simplest expression is just a constant. When SQL expressions get more complex, operator precedence can become an issue; in other words: what are the operator priority rules Of course, SQL has some precedence rules. For example, arithmetic operators always have precedence over comparison operators, and comparison operators have precedence over logical operators. However, it is highly recommended that you use parentheses in your complex SQL expressions to force a certain expression evaluation order, just as you would do in regular mathematics.

Oracle has added a lot of functionality to the SQL standard in the area of functions. This is definitely one of the reasons why Oracle SQL is so powerful. You can recognize SQL functions by their signature: they have a name, followed by one or more arguments (between parentheses) in a comma-separated list. You can use functions in expressions, in the same way that you can use operators. These are the six SQL function categories, based on their operand types: Numeric functions Alphanumeric functions Group functions Date functions Conversion functions Other functions