How To Get Data From Two Tables In Sql – Objectives After completing this lesson, you should be able to: Write SELECT statements to access data from multiple tables using a combination of equality and inequality. Use outer joins to view data that normally doesn’t meet the join criteria. Connect to the schedule using your autopilot
EMP DEPT EMPNO ENAME … DEPTNO KING 7698 BLAKE … 7934 MILLER DEPTNO DNAME LOC 10 ACCOUNTS NEW YORK 20 RESEARCH Dallas 30 SALES Chicago 40 OPERATIONS BOSTON EMPNO DEPNO LOCATION NEW YORK. CHIORGO 40 Data from multiple tables Sometimes it is necessary to use data from more than one table. In the example slide, the report contains data from two separate tables. EMPNO is located in the EMP table. DEPTNO is available in both EMP and DEPT tables. The LOC is located in the DEPT table. To create a report, you need to link the EMP and DEPT tables and get data from both. In the class management note slide, the DEPTNO column can appear from either the EMP or DEPT table.
How To Get Data From Two Tables In Sql
What is a login? Use joins to query data from multiple tables. Enter a WHERE clause to join. When the same column name appears in more than one table, type the table name before the column name. SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2; Specifying a join The join clause is used when data is needed from more than one table in the database. Rows in one table can be joined to rows in another table based on common values in corresponding columns, usually primary and foreign key columns. To display data from two or more related tables, type a simple join clause in the WHERE clause. In the syntax: table1.column represents the table and the column from which data is obtained table1.column1 = the clause that joins (or joins) the tables table2.column2 Recommendations When writing a SELECT statement that joins tables, before the name place the column. with the table name to improve the clarity and accessibility of the database. If the same column name exists in more than one table, the column name must precede the table name. Joining n tables requires at least (n-1) join conditions. Therefore, at least three joins are required to join four tables. This rule may not apply if your table has a linked primary key. In this case, more than one column is needed to define each row. For more information, see Oracle Server SQL Reference Manual, Release 8, SELECT.
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Join condition ignored Join condition is invalid All rows in the first table are joined with all rows in the second table. To avoid Cartesian products, always include a valid join condition in the WHERE clause. Cartesian Product When the join condition is false or omitted altogether, the result is the Cartesian Product, which shows all combinations of the string. All rows in the first table are joined to all rows in the second table. The Cartesian product usually produces many rows and the output is rarely useful. Unless you specifically need to join all rows in all tables, you should always include a valid join clause in the WHERE clause.
EMP (14 lines) DEPT (4 lines) EMPNO ENAME … DEPTNO KING 7698 BLAKE … 7934 MILLER DEPTNO DNAME LOC 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 FALES CHICAGO 40 OPERATIONS product K. “64 lines” Cartesian product (continued) If the join condition is omitted, the Cartesian derivative is generated. The sample slide shows the employee name and department name from the EMP and DEPT tables. Since no WHERE clause is specified, all rows in the EMP table (14 rows) are concatenated with all rows in the DEPT table (4 rows), resulting in 56 rows in the output. Show Class Control Log: l4cart.sql Purpose: Show implementation of Cartesian products. SQL SELECT ename, dname 2 FROM emp, dept; ENAME DNAME KING ACCOUNTING BLAKE RESEARCH … KING ACCOUNTING BLAKE RESEARCH 56 rows selected.
There are two main types of join terms: Parallel and parallel join methods These are: Outer joins Self joins Set operators Note: Set operators are not covered in this course. These are covered in the next SQL course. Note: Class Management Do not go into the details of all types of combinations. Explain each relationship one by one on the following slides.
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EMPNO Ename Deptno 7839 King 7839 Blake 7782 Clark 7782 Clark 7566 Jones 7654 ALLEN 7844 Smith … 14. Deptno Dname, New York 30 Sales Chicago 10 14 selected row. Equijoins Compare the DEPTNO column value in the EMP table with the DEPTNO values in the DEPT table to determine the employee’s department name. The relationship between the EMP and DEPT tables is equal, that is, the values of the DEPTNO column in both tables must be equal. Often this type of combination includes additions to primary and foreign keys. Note: Parallels are also called simple joins or internal joins. Note: Classroom Management Explain the use of a decision matrix in simplifying written composition. For example, if you wanted to display the names and department numbers of all the employees who work in the same department as Smith, you could start with a decision tree like this: SQL statements can now be easily organized by viewing the decision matrix . . . The columns of the SELECT statement are listed in the first column, the FROM clause in the second column, and the WHERE clause in the third column. Foreign key Primary key
SQL> SELECT emp.empno, emp.ename, emp.deptno, 2 dept.deptno, dept.loc 3 FROM emp, dept.loc WHERE emp.deptno=dept.deptno; EMPNO ENAME DEPTNO DEPTNO LOC 7839 KING NEW YORK 7698 BLAKE CHICAGO 7782 CLARK NEW YORK 7566 JONES DALLAS … 14 rows selected. Searching records with equivalent elements In the example of the slide: the SELECT clause specifies the names of the columns to be searched: employee name, employee number and department number, which are columns in the EMI table, department number, department name and location, the DEPT table and the FROM clause of two tables that are located by the database, defines: Table EMP DEPT Table DEPT The WHERE clause determines how the tables are joined: EMP.DEPTNO=DEPT.DEPTNO Since the DEPTNO column is common to both tables, it must be named with the table name to avoid ambiguity. be a prefix.
Use table prefixes to evaluate column names in more than one table. Improve performance with table prefixes. Separate columns with the same name but in different tables using column aliases. Using ambiguous column names To avoid ambiguity, you must specify column names in the WHERE clause with the table name. In addition to table prefixes, the DEPTNO column can come from either the DEPT table or the EMP table. A table prefix is required to execute the query. If there are no common column names between the two tables, there is no need to match the columns. However, you’ll do better by using a table prefix because you’ll tell the Oracle server exactly where to go to find the columns. The requirement to specify ambiguous column names also applies to columns that may be ambiguous in other clauses, such as SELECT or ORDER BY clauses. View Class Control Log: l4loc.sql Purpose: To display a SELECT clause without an alias.
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SQL> SELECT emp.empno, emp.ename, emp.deptno, 2 dept.deptno, dept.loc 3 FROM emp, dept. 4 WHERE emp.deptno=dept.deptno 5 AND dept.dname AS ‘FURUS’; Additional search criteria In addition to joins, you can have criteria in the WHERE clause. For example, King requires an additional WHERE clause to specify the employee number, name, department number, and department location. SQL> SELECT empno, ename, emp.deptno, loc 2 FROM emp, dept 3 WHERE emp.deptno = dept.deptno 4 AND INITCAP(ename) = ‘King’; EMPNO ENAME DEPTNO LOC 7839 KING NEW YORK
SQL> SELECT emp.empno, emp.ename, emp.deptno, 2 dept.deptno, dept.loc 3 FROM emp, dept.loc WHERE emp.deptno=dept.deptno; SQL> SELECT e.empno, e.ename, e.deptno, d.deptno, d.loc 3 FROM emp e, dept d 4 WHERE e.deptno=d.deptno; Table Aliases Assigning column names to table names can be time-consuming, especially when the table names are long. You can use table aliases instead of table names. Just as a column alias gives a different name to a column, a table alias gives a different name to a table. Table aliases help to reduce the size of SQL code, resulting in less memory usage. Notice how the table aliases are shown in the FROM clause of the example. Specify the full table name followed by a space and the table alias. The EMP table has a nickname
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