What does a question mark represent in SQL queries

Question

While going through some SQL books I found that examples tend to use question marks     in their queries  What does it represent

User · Answer

The   is an unnamed parameter which can be filled in by a program running the query to avoid SQL injection

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It s a parameter  You can specify it when executing query

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What you are seeing is a parameterized query  They are frequently used when executing dynamic SQL from a program   For example  instead of writing this  note  pseudocode    ODBCCommand cmd   new ODBCCommand  SELECT thingA FROM tableA WHERE thingB   7   result   cmd Execute     You write this   ODBCCommand cmd   new ODBCCommand  SELECT thingA FROM tableA WHERE thingB       cmd Parameters Add 7  result   cmd Execute     This has many advantages  as is probably obvious  One of the most important  the library functions which parse your parameters are clever  and ensure that strings are escaped properly  For example  if you write this   string s   getStudentName   cmd CommandText    SELECT   FROM students WHERE  name        s        cmd Execute     What happens when the user enters this   Robert    DROP TABLE students  --    Answer is here   Write this instead   s   getStudentName   cmd CommandText    SELECT   FROM students WHERE name      cmd Parameters Add s  cmd Execute     Then the library will sanitize the input  producing this    SELECT   FROM students where name    Robert     DROP TABLE students  --     Not all DBMS s use    MS SQL uses named parameters  which I consider a huge improvement   cmd Text    SELECT thingA FROM tableA WHERE thingB    varname  cmd Parameters AddWithValue   varname   7  result   cmd Execute

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The   is to allow Parameterized Query  These parameterized query is to allow type-specific value when replacing the   with their respective value    That s all to it   Here s a reason of why it s better to use Parameterized Query  Basically  it s easier to read and debug

User · Answer

It normally represents a parameter to be supplied by client

User · Answer

I don t think that has any meaning in SQL  You might be looking at Prepared Statements in JDBC or something  In that case  the question marks are placeholders for parameters to the statement

[sql] What does a question mark represent in SQL queries?

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