one, where
WHERE 列名 <比较运算符> 常量或列名
Commonly used comparison operators:
| operator | meaning |
|---|---|
| =, >,<,>=,<=, !=,> | Comparison of size |
| AND, OR, NOT | multiple conditions |
| IN | Determine the collection |
| BETWEEN AND | Determine the scope |
| IS NULL | empty value |
| LIKE | character match |
1. Compare size
Priority: NOT, AND, OR
can use parentheses to change the priority
[Example 8-18] Query the student number, class number and grades of students whose scores are higher than 90.
SELECT*
FROM sC
WHERE Grade>90;
[Example 8-19] Query the student ID and grades of the students whose grades are in [70.80].
SELECT Sno,Grade
FROM sC
WHERE Grade>=70 and Grade<=80;
[Example] Query the student number, course number and grades of elective C1 or C2 and the score is greater than or equal to 80 points.
SELECT Sno,Cno,gradeFROM sc
WHERE (Cno='C1' OR Cno='C2') AND grade>=80;
#或者
WHERE Cno='C1' AND grade>=80
OR Cno='C2'AND grade>=80
2. Query with IN
IN and NOT IN
[Example 8-20] Query the student number and class number of the class C1, C2 or C3.
SELECT Sno,Cno
FROM sc
WHERE Cno IN('C1, 'C2', 'C3') ;
3. Determine the scope (between and)
[Example 8-21] Query (包括75和80)the student number, class number and grades of the students whose grades are between 75 and 80 points.
SELECT Sno,Cno,Grade
FROM Sc
WHERE Grade BETWEEN 75 AND 80;
或者不在此范围 NOT BETWEEN 75 AND 80
4. Null query (IS NULL or IS NOT NULL)
Use the predicate IS NULL or IS NOT NULL
注意“IS NULL”不能用“= NULL”代替。
[Example 8-24] to query the student number and program number of the students who did not take the exam after the course selection.
SELECT Sno，Cno
FROM sc
WHERE Grade IS NULL;
5. String matching
Format:
attribute name LIKE '<matching string>'
attribute name NOT LIKE '<matching string>'
<matching string> = character constant + wildcard two kinds of wildcards:
%: represents任意长度a string (length can be 0)
_: represents any单个character
[Example] Query the student information in the S table whose surname is Li and whose name is three characters.
SELECT *
FROM Stu
WHERE Sname LIKE '李__';
[Example 8-26] Query the information of all students surnamed Li in the student table Stu.
SELECT *
FROM Stu
WHERESname LIKE '李%';
[Supplementary] Query the student number and name of the student with "Mei" in the name.
SELECT Sno,Sname
FROM Stu
WHERE Sname LIKE '%梅%';
[Supplementary] Query the student number and name of the student whose second character is "Mei"
SELECT Sno,Sname
FROM Stu
WHERE Sname LIKE '_梅%';
6. Define the escape character
ESCAPE'<escape character>'
such as: ESCAPE '/'
[Supplementary] Query the course number, course name and credits whose course name starts with 'DB_'.
SELECT Cno, Cname, Ccredit
FROM C
WHERE Cname LIKE 'DB/_%'
ESCAPE '/';