- 2.1, the control function capitalization
- 2.2, character control function
- 2.3 Digital function
- 2.4, date function
- 2.5, the conversion function
- 2.6, generic function
- 2.7, the conditional expression
- 3.1, Cartesian product
- Alias 3.2, table
- 3.3, a plurality of connection tables
- 3.4, non-equivalent connection
- 3.5, the inner and outer joins
- 3.6, natural connection
- 3.7, ON clause to create a connection
- 3.8, since the connection
- 3.9, left (and right) outer join, and full outer joins
Introduces SQL function
One-way function and the multi-line function
One-way function
---- INITCAP (ename) first letter capitalized, upper (upper case), lower (lowercase) ----
the SELECT A. * A emp from the WHERE INITCAP (a.ename) = 'James' and Upper (a.ename) = 'JAMES'
example:
---- names include all alternative A is 0 ----
SELECT Replace (a.ename, 'A', '0') from EMP ename A
---- salary 5, if not from the left 5 start with added 0 ----
SELECT LPAD (a.sal, '. 5', '0') A SAL from EMP
- DUAL is a 'pseudo-table', it can be used to test the expression and function
---- round: rounding ----
SELECT round (45.625, 2) from Dual; -> 45.63
----trunc：截断----
select trunc(45.625, 2) from dual; -->45.62
----mod：求余----
select mod(500, 300) from dual; -->200
- Date data type in Oracle actually contains two values: the date and time.
- SYSDATE function returns: the date, time, for example: select sysdate from dual; -> 2020/3/12 00:00:00
- Date Functions
5, the transfer function
-
Oracle automatically following conversion (implicit data type conversions)
For example: select '11' + 2 from dual; -> 13; -
The display converter
---- TO_CHAR date string rotation, string digital-to- ----
SELECT TO_CHAR (SYSDATE, 'the MM-dd-YYYY'), TO_CHAR (2121.13, 'L000,999.99') from Dual; -> 2020-03-12, ¥ 002,121.13
---- TO_DATE string transfer date ----
SELECT TO_DATE ( '2020-03-12', 'the MM-dd-YYYY') from Dual; -> 2020 /. 3 / 12 is
---- TO_DATE string-to-digital ----
SELECT to_number ( '¥ 002,121.13', 'L000,999.99') from Dual; -> 2121.13
-
Date format element
6, generic function -
NVL (expr1, expr2) -----> If expr1 is null, then the value of expr2 choice, otherwise choose their own
---- query table does not have the staff department employee number, department number to write no department ----
the SELECT a.ename, NVL (the TO_CHAR (a.mgr), 'no department') job from emp a;
- NVL2 (expr1, expr2, expr3) -----> If the selection is not empty expr1 expr2, if the selection is empty expr3
---- queries sector no bonus prize of 1, plus there's a bonus ---- 0.5
the SELECT a.ename, NVL2 (a.comm, a.comm + 0.5, 1) from emp A
- NULLIF (expr1, expr2) -----> equal returns NULL, ranging from the return expr1
- COALESCE (expr1, expr2, ..., exprn) -----> advantages compared with COALESCE COALESCE NVL in that a plurality of values can be processed simultaneously alternately. If the first expression is empty, the next expression returns on other parameters COALESCE
- Using the IF-THEN-ELSE logic in SQL statements
- There are two ways:
the CASE expression
DECODE function
----- department number for employees to query information 10, 20, 10 printing sector wages 1.1 times, 1.2 times the wages in other sectors print (the CASE) ----
the SELECT a.ename, a.deptno, Case A .deptno when 10 then a.sal * 1.1 else a.sal * 1.2 end new_sal from emp a;
-----（DECODE）----
select a.ename,a.deptno,decode(a.deptno,10,a.sal * 1.1,a.sal * 1.2) new_sal from emp a ;
- A training practice
---- 1. Print out the "2020 March 12 09:05:08" the current system date and time format (Note: Use double quotes to add character to date)
the SELECT the TO_CHAR (sysdate, 'YYYY' in "MM" month "DD" day "HH: MI: SS ') from dual; ----> 2020 March 12 09:05:08
---- 2, the format numbers: 1,234,567.89 to 1,234,567.89
SELECT TO_CHAR (1234567.89, '999,999,999.99') from Dual;
---- 3, to digital string, if no special characters in the string, an implicit conversion may be:
SELECT '1234567.89' from + 100 Dual;
---- 4, if the special characters string, e.g. "1,234,567.89", implicit conversion is not necessary to use to_number () to complete the
select to_number ( '1,234,567.89', ' 999,999,999.99') + 100 from dual; - ---> 1,234,667.89
---- 5, the date for the query as a query, generally use to_date () to a string to date, so do not have to focus on the date format
select a.ename, a.hiredate from emp a where a.hiredate = to_date ( '1981-2-20', ' yyyy-mm-dd');
---- 6, the end of each month inquiry penultimate two days recruits information
. Select a * from emp a where a.hiredate = last_day (a.hiredate) - 2;
---- 7. calculated employees salary
select a *, a.sal * 12 + nvl (a.comm, 0) salary from emp a.;
Multi-table query
-
TABLE EMPLOYEES (20 rows), Table DEPARTMENTS (8 lines), the Cartesian set: 20x8 = 160 rows
-
Descartes rally produced under the following conditions:
Connection condition omitted
join condition is invalid
in all rows in all tables are connected to each other
- How to avoid Cartesian set:
May be added in effective connection conditions WHERE.
- Using aliases, simplify the query. Use table prefix can improve the efficiency.
Such as:
SELECT e.employee_id,
e.last_name,
e.department_id,
d.department_id,
d.location_id
FROM employees e, departments d
WHERE e.department_id = d.department_id
- Connecting the n-tables, at least n-1 connections conditions. For example: three connection table, at least two connection conditions.
- Staff query table (emp) in wages belong to the pay scale table (sal_grades) in the first few files:
select a.ename, a.sal, b.level
from emp a, sal_grades b
where a.sal between b.low_sal and hight_sal;
-
En :
combined with two or more rows of the same column of a table, the result set does not contain a table and another table rows that do not match -
External connection :
two tables satisfy the join condition in addition return line also returns the left (or right) in the table row condition is not satisfied during the connection, this connection is called a left (or right) outer join. No matching row, the corresponding result table is empty (NULL). WHERE clause conditions are similar to the outer connecting the internal connection, but is connected to the back of the list does not match the condition to increase the outer connecting line operator, i.e., with parentheses plus sign (+) of
Example (right outer join):
select * from emp a, dept b where a.deptno(+) = b.deptno;
- NATURAL JOIN clause creates the equivalent connection with the conditions listed in the two tables with the same name. In the table data query to satisfy the equivalent conditions. If only the same column name and different data types, an error occurs.
- Natural join condition is listed as a connection with the same name of. ON clause can specify additional connection conditions. The connection condition is separate from other conditions. ON clause makes statements higher legibility.
- ON clause to create a multi-table joins
SELECT employee_id, city, department_name
FROM employees e
JOIN departments d
ON d.department_id = e.department_id
JOIN locations l
ON d.location_id = l.location_id;
- Connect to their own
E.g:
select m.*
from employees e, employees m
where e.manager_id = m.employee_id and e.last_name = 'Chen'
-
During the connection the two tables in addition to the return line further satisfy the join condition to return the left (or right) table row condition is not satisfied, this connection is called a left (or right) outer join.
-
During the connection the two tables in addition to the return line further satisfy the join condition in the return line condition is not satisfied two tables, this connection is called a full external connection.
