Getting Started with SQL Server 7.0 (2)

Accessing data
select statement
1. Select all columns in the table
"*" has special meaning for Select statement. It specifies all columns in the table without listing the column names. The order of the columns is the same as in the table.
2． Select different values
If the columns in the selected list have duplicate values, the "distinct" keyword can be used to ignore the duplicate values.
Note: If there is more than one column in the Select list, the distinct keyword is effective for them all. If a column has duplicate values ​​and the values ​​in other columns are unique, the column with duplicate values ​​is included in the results.
3. Rename columns in the result data set
The default name of the column in the result is the column name in the source database. Users can replace the default column name with their own specified column name.
Select AuthorFirstName = au_fname from authors
--Change the column name from "au_fname" to "AuthorFirstName"
4． Select calculated values ​​
You can include calculated values ​​or constants in the Select list. A calculated value is calculated based on an arithmetic expression and can be contained in one or more columns in the table.
Select totalSale = PRice * ytd_sales from titles

Conditional selection (Where clause)
Use a comparison or logical operation to specify filter conditions in the Where clause to generate the desired rows in the table.
1. Comparison operations
Comparison operations can compare numerical, character and date data and return TRUE or FALSE.
Comparison operators
>(greater than)
=(equal to)

(not equal to)
>=(greater than or equal to)

! = (not equal to)
!!> (not greater than)


2. Logical operations
Logical operations test whether certain conditions are correct and return TRUE or FALSE based on the test results.
· LIKE returns TRUE if the operation is the same as the specified string, and the specified string can also contain wildcard characters.
The like operation with wildcard characters is more useful.
  · “%” specifies that all strings can replace the position of the character “%”. Any specified string before or after "%" is treated as a constant. For example: "New%" means all strings starting with "New", "%New" means all strings ending with "New".
· "_" specifies that any single character can be substituted into the position of "_". This is useful in the case of similar characters that differ by only one character.
  · “[]” specifies that the characters defined in square brackets should be used instead of one character. The square brackets may be independent characters (such as [ahg]) or character ranges (such as [c-i]).
· "[^]" specifies that the characters after "^" in square brackets are characters that cannot be substituted. It can be an independent character (such as [ahg]) or a character range (such as [c-i]).
Note: These wildcard characters can be combined in an expression.
· BETWEEN Returns TRUE if the operand is within the provided range.
BETWEEN can be used with the NOT operator, and returns TRUE if the data is outside the specified range.
Note: The BETTEN operator specifies a range. For example, BETWEEN A and B means all values ​​between A and B, inclusive.
· IN The IN operator returns TRUE if the operand matches any specified value. The specified value can be a constant list of values, or it can be generated by another query (called a subquery).
The IN operator can also be used in conjunction with the NOT operator. If the data is not in the specified value, it returns TRUE.
· AND It combines two expressions, if both expressions are TRUE, then it returns TRUE, otherwise it returns FALSE.
· OR It combines two expressions, if one of the two expressions is TRUE, then it returns TRUE, if both expressions are FALSE, it returns FALSE.
· NOT It negates the value of a logical operation. It can be used in conjunction with most logical operations to negate the value returned by the operation.
· SOME|ANY These two operations have the same format. They compare the operand to a specified set of values. It returns TRUE if comparison of the operand with any specified value returns TRUE. The comparison operation can be any comparison operation defined by SQL Server. The specified list of values ​​can be produced by another query.
· ALL The ALL operation has a similar format to SOME and ANY. The difference is that it only returns TRUE if the comparison of all operands with the specified value returns TRUE.

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