SQL Fundamentals: Basic SELECT statement基本的select语句

Posted 2020-09-26

Basic SELECT statement基本的select语句 The basic syntax for a SELECT statement is presented below. SELECT语句的基本语法如下。 |：多选一 []：可选择的内容 {}:多选一 没有被{}括起来的是必选 SELECT [DISTINCT | ALL] {* | select_list} FROM {table_name [alias] | view_name}     [{table_name [alias] | view_name}]...  [WHERE condition] [GROUP BY condition_list] [HAVING condition] [ORDER BY {column_name | column_# [ ASC | DESC ] } ...   In its simplest form, a SELECT statement must include the following: SELECT The SELECT clause is mandatory and carries out the relational project operation. 选择子句是强制性的，并执行关系项目操作。 SELECT identifies the columns to be displayed. A SELECT clause, which specifies the columns to be displayed. SELECT标识要显示的列。 FROM The FROM clause is also mandatory. It identifies one or more tables and/or views from which to retrieve the column data displayed in a result table. FROM identifies the table containing those columns. A FROM clause, which identifies the table containing the columns that are listed in the SELECT clause. 它标识包含SELECT子句中列出的列的表。   In the syntax: SELECT Is a list of one or more columns * Selects all columns DISTINCT Suppresses(阻止) duplicates(重复) Column | expression Selects the named column or the expression列名或表达式 alias Gives the selected columns different heading(标题) FROM table Specifies the table containing the columns Note: Throughout this course, the words keyword, clause, and statement are used as follows: A keyword(关键字) refers to an individual（独特的） SQL element（有特殊含义的SQL元素） ——for example,SELECT and FROM are keywords. A clause(子句) is a part of a SQL statement（Select语句的一个组成部分） ——for example, SELECT emplovee_id, last_name , and so on A statement(语句) is a combination（组合） of two or more clauses（Select语句是两个或者多个子句的组合） ——for example, SELECT * FROM employees（SELECT *叫一个子句，FROM employees叫一个子句） 注意：习惯将关键字写成大写 WHERE The WHERE clause is optional and carries out the relational select operation. It specifies which rows are to be selected.  GROUP BY The GROUP BY clause is optional. It organizes data into groups by one or more column names listed in the SELECT clause. HAVING The optional HAVING clause sets conditions regarding which groups to include in a result table. The groups are specified by the GROUP BY clause. ORDER BY The ORDER BY clause is optional. It sorts query results by one or more columns in ascending or descending order.   Selecting All Columns: SELECT * FROM departments；   从departments表中选择所有的行rows. 每个行要显示所有列column.   Selecting Specific Columns: SELECT department_id, location_id FROM departments； 从departments表中选择指定行.   Write SQL Statements SQL statements are not case sensitive. SQL语句不区分大小写 SQL statements can be entered on one or more lines. 可以在一行或多行上输入sql语句 Keywords cannot be abbreviated(缩写) or split across lines. 关键字不能缩写或跨越多行 Clauses are usually placed on separate lines. 子句通常放在单独的行上 Indents(缩进) are used to enhance redability. 缩进是用来增强可读性 In SQL Developer, SQL statements can be optionally terminated by semicolon(;).Semicolons are required when you execute multiple SQL statements. In SQL Developer,SQL语句可以随意终止的分号（；） 当你执行多条SQL语句时分号是必需的。 In SQL*Plus, you are required to end each SQL statement with a semicolon(;). In SQL*Plus,，你需要用分号（；）结束每个SQL语句。 Chose the statements which correctly specify a rule to write a SQL statement SQL statements are case sensitive Keywords can be abbreviated to build a standard SQL statements are case in-sensitive clauses must be placed together Answer: C.SQL statements are not case sensitive.   Column Heading Defaults: 默认的列标题（表的第一行）: SQL Developer -Default heading alignment: Left-aligned(左对齐) -Default heading display: Uppercase(大写) SQL *Plus -Character and Data column headings are left-aligned -Number column headings are right-aligned -Default heading display : Uppercase     Arithmetic expressions and NULL values in the SELECT statement SELECT语句中的算术表达式和空值   首先介绍显示表结构的命令 DESCRIBE command 描述命令：显示表结构 The structural metadata of a table may be obtained by querying the database for the list of columns that comprise it using the DESCRIBE command. It will list the used column names, their null property and data type. Syntax: DESC[RIBE] [SCHEMA].object name For example, SQL> desc emp;  Name                                      Null?    Type  ----------------------------------------- -------- ----------------------------  EMPNO                                     NOT NULL NUMBER(4)  ENAME                                              VARCHAR2(10)  JOB                                                VARCHAR2(9)  MGR                                                NUMBER(4)  HIREDATE                                           DATE  SAL                                                NUMBER(7,2)  COMM                                               NUMBER(7,2)  DEPTNO                                             NUMBER(2)   will display the EMPLOYEE table structure i.e. columns, their data types, precision and nullable property.     An arithmetic expression can be creaeted using the column names, operators and constant values to embed an expression in a SELECT statement. 算术表达式可以使用列名称、运算符和常量值嵌入一个SELECT语句中的表达创建。 The operator applicable to a column depends on column‘s data type. 适用于列的运算符依赖于列的数据类型。 For example, arithmetic operators will not fit for character literal values. 例如，算术运算符不适合字符面值。    For example, SELECT employee_id, sal * 12 ANNUAL_SAL FROM employees; The above query contains the arithmetic expression (sal * 12) to calculate annual salary of each employee. 上面的查询包含算术表达式（sal* 12）来计算每个雇员的年薪。   Arithmetic Expressions 算术表达式 You may need to modify the way in which data is displayed, or you may want to perform calculations, or look at what-if scenarios. All these are possible using arithmetic expressions. 你可能需要修改数据的显示方式，或想执行计算，通过使用算术表达式可以实现. An arithmetic expression can contain column names, constant numeric values, and the arithmetic operators. 一个算术表达式可以包含列名、常量数字值和算术操作符. Create expressions with number and date data by using arithmetic operators.   Arithmetic operators 算术运算符 The slide lists the arithmetic operators that are available in SQL. You can use arithmetic operators in any clause of a SQL statement(except the FROM clause) 除了FROM子句，可在SELECT的任何其他子句中使用算术操作符. Note: With the DATE and TIMESTAMP data types, you can use the addition and subtraction. 对DATA和TIMESTAMP数据类型，只能使用+ -操作符.   Operators act upon the columns (known as operands) to result into a different result. In case of multiple operators in an expression, the order of evaulation is decided by the operator precedence. Here are the elementary rules of precedence - Multiplication and division occur before Addition and Subtraction. Operators on the same priority are evaluated from left to right. Use paretheses to override the default behavior of the operators. Below table shows the precedence of the operators, in such cases. Precedence Level Operator Symbol Operation Description   Operator         Precedence         Addition + Lowest Subtraction -         Lowest         Multiplication *         Medium         Division /         Medium Brackets ( )         Highest          Examine the below queries (a), (b), and (c) SQL> SELECT 2*3 FROM DUAL;        2*3 ---------- SQL> SELECT ename,sal,sal+200 FROM emp; ENAME             SAL    SAL+200 ---------- ---------- ---------- SMITH             800       1000 WARD             1250       1450   SQL> SELECT ename,sal,sal+(comm*sal) FROM emp; ENAME             SAL SAL+(COMM*SAL) ---------- ---------- -------------- SMITH             800 WARD             1250         626250 Query (a) multiplies two numbers, while (b) shows addition of $1500 to salaries of all employees. Query (c) shows the addition of commission component to employee‘s salary. As per the precedence, first commission would be calculated on the salary, and then added to the salary.   注意，如上sal+200这个列是运算列，并不是真正存在的列，这里可以对这个列设定一个列别名.   Column Alias 列别名（可以改变列标题） Renames a column heading Is useful with calculations Immediately follows the column name(There can also be the optional AS keyword between the column name and the alias.) 紧跟列名（也可以在列名和别名之间选择关键字） Requires double quotation（双引号） marks if it contains spaces（空格） or special characters（特殊字符）, or if it is case-sensitive(区分大小写). 如果别名列中有空格，或特殊字符，或区分大小写，这个时候这个列标题要用双引号引起来.   An alias is used to rename a column or an expression during display. The alias to a column or an expression appears as the heading in the output of a query. It is useful in providing a meaningful heading to long expressions in the SELECT query. By default, the alias appears in uppercase in the query output without spaces. To override this behavior, alias must be enclosed within double quotes to preserve the case and spaces in the alias name. SELECT price * 2 as DOUBLE_PRICE, price * 10 "Double Price" FROM products; DOUBLE_PRICE        Double Price ------------        ------------ 39.9                        39.9 60                        60 51.98                        51.98   Concatenation operators 串联操作符|| A concatenation（串联） operator: Links columns or character strings to other columns 将列或字符串连接起来作为一个单独的列作为输出. Is represented by two vertical bars（||） Creates a resultant（组合） column that is a character expression.   将列或字符串连接起来，创建一个组合列.   Concatenation operator can be used to join two string values or expressions in a SELECT query. The double vertical bar symbol is used as string concatenation operator. It is applicable only for character and string column values resulting into a new character expression. Example SQL> SELECT ename,job,ename||job as "Employees" FROM emp;   ENAME      JOB       Employees ---------- --------- ------------------- SMITH      CLERK     SMITHCLERK The above query shows concatenation of two character literals values.   Literals 字面量/常量 A literal is a character, a number, or a date that is included in the SELECT statement. 常量可以是字符串，数字，日期. Data and character literal values must be enclosed within single quotation (单引号)marks. 日期和字符串常量必须用单引号引起来. Each character string is output once for each row returned. 为返回的每一行打印一次.   单引号‘引起来的是字面量. 单引号为定界符.   Any hard coded value, which is not stored in database, in the SELECT clause, is known s Literal. It can be number, character, or date value. Character and date values must be enclosed within quotes. Consider the below SQL queries.examples of using literals of different data types in SQL queries. The query below uses two character literals to join them together. SQL>  SELECT ‘ORACLE‘||‘ CERTIFICATION‘ FROM DUAL The query below uses character literals to pretty print the employee‘s salary. SQL> SELECT first_name ||‘earns‘|| salary||‘ as of ‘|||sysdate FROM employees SQL> SELECT ename,job,ename||‘job is‘||job as "Employees" FROM emp;   ENAME      JOB       Employees ---------- --------- ------------------------- SMITH      CLERK     SMITHjob isCLERK   Quote Operator 引用运算符 Specify your own quotation mark （引号）delimiter. Select any delimiter（定界符）. Increase readability and usability（适用性）. Q运算符适用于字符串本身包含单引号的情况. 使用Q操作符可以选择自己的引号定界符. 你可以选择任何方便的定界符，[],{},(),<>. The quote operator is used to specify the quotation mark delimiter of your own. You can chose a convenient delimiter, depedning on the data. 如下例子中，使用q操作符，中括号[]被作为字符串的定界符，里面的单引号‘就是普通的字符. SELECT         department_name|| ‘ Department‘ ||q‘[‘s Manager Id: ]‘|| manager_id FROM departments; SQL> SELECT ename,job,ename||q‘[‘s job is]‘||job as "Employees" FROM emp;   ENAME      JOB       Employees ---------- --------- ---------------------------- SMITH      CLERK     SMITH‘s job isCLERK   NULL空值 Null is a value that is unavailable, unassigned(未赋值), unknown, or inapplicable（不适用的）. Null is not the same as zero（数值类型的0） or a blank space（字符类型的空格）. 包含NULL值的算术表达式，其结果还是NULL.     If a column doesn‘t has a definite value, it is considered as NULL. NULL value denotes unknown or unavailable. It is not zero for numeric values, not blank space for character values. Columns with NULL value can be selected in a SELECT query and can be the part of an arithmetic expression. Any arithmetic expression using NULL values results into NULL. For this reason, columns with NULL value must be handled differently by specifying their alternate values using Oracle supplied functions like NVL or NULLIF. SQL> SELECT NULL + 1000 NUM FROM DUAL; NUM --------         Duplicate Rows The default display of queries is all rows, including duplicate rows. DISTINCT Keyword:Suppresses(阻止) duplicates(重复) 这里可以选择多个列，但是他会取多个列的组合的重复，如列1的前三行为1，2，2；列2的前三行为1，3，4，会取出第一行，显示第二行和第三行. If the data is expected to have duplicate results, use DISTINCT keyword to eliminate duplicates and diplay only the unique results in the query output. Only the selected columns are validated for duplication and the rows will be logically eliminated from the query output. To be noted, the DISTINCT keyword must appear just after the SELECT clause. The simple query below demonstrates the use of DISTINCT to display unique department ids from EMPLOYEES table. DISTINCT后可以跟多个列，去掉多个列的组合完全一样的重复行. SQL> SELECT DISTINCT deptno FROM EMP;       DEPTNO ----------         30         20         10 Question：Which of the following clause is used to suppress duplicates in a SELECT statement? INTERSECT DUPLICATE DISTINCT UNIQUE Answer: C, D. Duplicate data can be restricted with the use of DISTINCT or UNIQUE in the SELECT statement.

1. Identify the capabilities of SELECT statement. Projection Selection Data Control Transaction Answer: A, B. The SELECT statement can be used for selection, projection and joining. 2. Determine the capability of the SELECT statement demonstrated in the given query. SELECT e.ename, d.dname FROM   emp e, dept d WHERE e.deptno = d.deptno AND    e.sal > 1000; Selection Filtering Joining Projection Answer: A, C, D. Projection is including only the required columns in query, while Selection is selecting only the required data. Joining means combining two tables together through a connecting column. 3. Which of the following clause is used to suppress duplicates in a SELECT statement? INTERSECT DUPLICATE DISTINCT UNIQUE Answer: C, D. Duplicate data can be restricted with the use of DISTINCT or UNIQUE in the SELECT statement. 4. Chose the statements which correctly specify a rule to write a SQL statement SQL statements are case sensitive Keywords can be abbreviated to build a standard SQL statements are case in-sensitive clauses must be placed together Answer: C.SQL statements are not case sensitive. 5. Determine the output of the below query - SELECT ‘5+7‘ FROM dual; 12 5+7 5 7 Answer: B.Oracle treats the values within double quotes as string expressions. 6. Write a query to display employee details (Name, Department, Salary and Job) from EMP table. SELECT ename, deptno, sal, job FROM emp; SELECT * FROM emp; SELECT DISTINCT ename, deptno, sal, job FROM emp; SELECT ename, deptno, sal FROM emp; Answer A.Select the required from the tables each separated by a comma. 7. Which of the below queries displays employees‘ name and new salary after the increment of 1000? SELECT ename, sal FROM emp; SELECT ename, sal=sal+1000 FROM emp; SELECT ename, sal+1000 FROM emp; SELECT ename, 1000 FROM emp; Answer: C. Basic arithmetic calculations can be done using the columns in SELECT statements. 8. Determine the output of the below query SELECT 36/2-5*10 FROM dual; 130 -32 -120 175 Answer: B. Multiplication and Division occur before addition and subtraction. 9. Determine the output of the below query SELECT (100-25)/15*(20-3) FROM dual; 0.294 -85 63.67 85 Answer: D. Expression within the brackets are executed before the divisions and multiplications in the expression. 10. Chose the statements which correctly define a NULL value. NULL is a special value with zero bytes NULL is no value or unknown value NULL is represented by a blank space NULL is not same as zero Answer: B, D.NULL is NO VALUE but neither same as zero nor as blank or space character. 11. Determine the output of the below query SELECT sal + NULL FROM emp WHERE empno = 7369; sal + NULL NULL 0 1250 Answer: B. Any arithmetic operation with NULL results in NULL. 12. Which of the below statements define column alias correctly? A column alias renames a column heading A column alias is an alternate column in a table A column alias can be specified during table definition A column alias immediately follows the column or expression in the SELECT statement Answer: A, D. Column Alias can be used to name an expression in the SELECT statement. 13. Specify the column alias NEWSAL for the expression containing salary in the below SQL query SELECT ename, job, sal + 100 FROM emp; (sal + 100) AS NEWSAL (sal + 100) NEWSAL (sal + 100) IS NEWSAL sal + 100 IS NEWSAL Answer: A, B.Use ‘AS‘ to signify new alias to a column expression. 14. Specify the column alias "New Salary" for the expression containing salary in the below SQL query SELECT ename, job, sal + 100 FROM emp; (sal + 100) AS New Salary (sal + 100) "New Salary" (sal + 100) IS New Salary sal + 100 as "New Salary" Answer: B, D. Column alias with space and special characters must be enquoted within double quotes. 15. Which command is used to display the structure of a table? LIST SHOW DESCRIBE STRUCTURE Answer: C.DESCRIBE is used to show the table structure. 16. Predict the output when below statement is executed in SQL* Plus? DESC emp Raises error "SP2-0042: unknown command "desc emp" - rest of line ignored." Lists the columns of EMP table Lists the EMP table columns, their data type and nullity Lists the columns of EMP table along with their data types Answer: C. DESCRIBE is used to show the table structure along with table columns, their data type and nullity 17. Which of the below statements are true about the DESCRIBE command? It can be used in SQL*Plus only It can be used in both SQL*Plus as well as SQL Developer It doesn‘t works for object tables It doesn‘t works for SYS owned tables Answer: B. 18. Which of the below alphanumeric characters are used to signify concatenation operator in SQL? + || - :: Answer: B.In SQL, concatenation operator is represented by two vertical bars (||). 19. Which of the below statements are correct about the usage of concatenation operator in SQL? It creates a virtual column in the table It generates a character expression as the result of concatenation of one or more strings It creates a link between two character columns It can be used to concatenate date expressions with other columns Answer: B, D. Concatenation operator joins two values as an expression. 20. Predict the output of the below query SELECT ename || NULL FROM emp WHERE empno = 7369 SMITH SMITH NULL SMITHNULL ORA-00904: "NULL": invalid identifier Answer: A. Concatenation with NULL results into same value. 21. Predict the output of the below query SELECT 50 || 0001 FROM dual 500001 51 501 5001 Answer: C. The leading zeroes in the right operand of expression are ignored by Oracle. 22. You execute the below query SELECT e.ename||‘ departments‘s name is:‘|| d.dname FROM emp e, dept d where e.deptno=d.deptno; And get the exception - ORA-01756: quoted string not properly terminated. Which of the following solutions can permanently resolve the problem? Use double quote marks for the literal character string Use [q] operator to enquote the literal character string and selecting the delimiter of choice Remove the single quote mark (apostrophe) from the literal character string Use another delimiter to bypass the single quote apostrophe in the literal string Answer: B. The [q] operator is used to enquote character literals with a quote. 23. Which of the below SELECT statement shows the correct usage of [q] operator? SELECT e.ename || q‘[department‘s name is]‘|| d.dname    FROM emp e, dept d    WHERE e.deptno = d.deptno; SELECT e.ename || q[‘department‘s name is‘]|| d.dname    FROM emp e, dept d    WHERE e.deptno = d.deptno; SELECT e.ename || q[department‘s name is]|| d.dname    FROM emp e, dept d    WHERE e.deptno = d.deptno; SELECT e.ename || q‘(department‘s name is)‘|| d.dname    FROM emp e, dept d    WHERE e.deptno = d.deptno; Answer: A 24. Which of the below SELECT statement is used to select all columns of EMP table? SELECT ALL FROM emp SELECT # FROM emp SELECT * FROM emp SELECT empno,ename,deptno,sal,job,mgr,hiredate FROM emp Answer: C. The character ‘*‘ is used to select all the columns of the table. 25. Which of the below SQL query will display employee names, department, and annual salary? SELECT ename, deptno, sal FROM emp; SELECT ename, deptno, sal + comm FROM emp; SELECT ename, deptno, (sal * 12) Annual_Sal FROM emp; Annual salary cannot be queried since the column doesn‘t exists in the table Answer: C. Use numeric expressions in SELECT statement to perform basic arithmetic calculations.