过多使用3个以上表的内部联接

Question

Good Day,

I have 4 tables on my DB (not the actual name but almost similar) which are the ff: employee,education,employment_history,referrence employee_id is the name of the foreign key from employee table.

Here's the example (not actual) data:

**Employee**
ID       Name      Birthday     Gender           Email
1     John Smith  08-15-2014     Male     johnsmith@extension.com
2     Jane Doe    00-00-0000    Female    janedoe@extension.com
3     John Doe    00-00-0000     Male     johndoe@extension.com

**Education**
Employee_ID     Primary            Secondary             Vocation
1              Westside School    Westshore H.S       SouthernBay College
2              Eastside School    Eastshore H.S       NorthernBay College
3              Northern School    SouthernShore H.S   WesternBay College

**Employment_History**
Employee_ID      WorkOne         StartDate     Enddate
1              StarBean Cafe    12-31-2012    01-01-2013
2              Coffebucks Cafe  11-01-2012    11-02-2012
3              Latte Cafe       01-02-2013    04-05-2013

Referrence
Employee_ID     ReferrenceOne         Address        Contact
1               Abraham Lincoln   Lincoln Memorial  0000000000
2               Frankie N. Stein   Thunder St.       0000000000
3               Peter D. Pan      Neverland Ave.    0000000000

NOTE: I've only included few columns though the rest are part of the query.

And below are the codes I've been working on for 3 consecutive days:

$sql=mysql_query("SELECT emp.id,emp.name,emp.birthday,emp.pob,emp.gender,emp.civil,emp.email,emp.contact,emp.address,emp.paddress,emp.citizenship,educ.employee_id,educ.elementary,educ.egrad,educ.highschool,educ.hgrad,educ.vocational,educ.vgrad,ems.employee_id,ems.workOne,ems.estartDate,ems.eendDate,ems.workTwo,ems.wstartDate,ems.wendDate,ems.workThree,ems.hstartDate,ems.hendDate 

FROM employee AS emp INNER JOIN education AS educ ON educ.employee_id='emp.id' INNER JOIN employment_history AS  ems ON ems.employee_id='emp.id' INNER JOIN referrence AS  ref ON ref.employee_id='emp.id' 

WHERE emp.id='$id'");

Is it okay to use INNER JOIN this way? Or should I modify my query to get the results that I wanted? I've also tried to use LEFT JOIN but still it doesn't return anything .I didn't know where did I go wrong. You see, as I have thought, I've been using the INNER JOIN in correct manner, (since it was placed before the WHILE CLAUSE ). So I couldn't think of what could've possible went wrong.

Do you guys have a suggestion? Thanks in advance.

Answer 1

I suggest you inspect your use of string literals, such as 'emp.id' in the join predicates.

I suspect you're not intending to compare to a string literal, but what you want is to compare to an expression that references a column in another table.

The predicate in your query compares the employee_id column to a string literal, a constant that contains six characters. It's the single quotes enclosing emp.id that makes MySQL see that as a literal here:

    ON educ.employee_id = 'emp.id'
                          ^      ^

I suspect you want to compare the employee_id column to the id column from the employees table,. The absence of single quotes makes MySQL see emp and id as identifiers here:

    ON educ.employee_id = emp.id
                          ^^^ ^^

If you need to "escape" identifiers in MySQL, use the backtick character, not a single quote. And enclose each identifier separately, for example:

    ON `educ`.`employee_id` = `emp`.`id`
       ^    ^ ^           ^   ^   ^ ^  ^

---

I have another suggestion too. I suggest you consider using the mysqli or PDO interface instead of the deprecated mysql interface.

I also suggest you consider possible SQL Injection vulnerabilities, including "unsafe" values as part of the SQL text. For example, consider what this string evaluates to:

   "... emp.id='$id'";

when $id variable contains a value such as 1' OR '1'='1