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This chapter is from the book

This chapter is from the book

SQL: Visual QuickStart Guide, 3rd EditionSQL: Visual QuickStart Guide, 3rd Edition

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This chapter is from the book

This chapter is from the book

SQL: Visual QuickStart Guide, 3rd EditionSQL: Visual QuickStart Guide, 3rd Edition

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Eliminating Duplicate Rows with DISTINCT

Columns often contain duplicate values, and it’s common to want a result that lists each duplicate only once. If I type Listing 4.6 to list the states where the authors live, the result, Figure 4.6, contains unneeded duplicates. The DISTINCT keyword eliminates duplicate rows from a result. Note that the columns of a DISTINCT result form a candidate key (unless they contain nulls).

Listing 4.6. List the states in which the authors live. See Figure 4.6 for the result.

SELECT state
  FROM authors;

Figure 4.6 Result of Listing 4.6. This result contains unneeded duplicates of CA and NY.

state
-----
NY
CO
CA
CA
NY
CA
FL

To eliminate duplicate rows:

  • Type:

    SELECT DISTINCT columns
  FROM table;

    columns is one or more comma-separated column names, and table is the name of the table that contains columns (Listing 4.7 and Figure 4.7).

    Listing 4.7. List the distinct states in which the authors live. The keyword DISTINCT eliminates duplicate rows in the result. See Figure 4.7 for the result.

    SELECT DISTINCT state
  FROM authors;

    Figure 4.7 Result of Listing 4.7. This result has no CA or NY duplicates.

    state
-----
NY
CO
CA
FL

Tips

  • If the SELECT DISTINCT clause contains more than one column, the values of all the columns combined determine the uniqueness of rows. The result of Listing 4.8 is Figure 4.8, which contains a duplicate row that has two columns. The result of Listing 4.9 is Figure 4.9, which eliminates the two-column duplicate.

    Listing 4.8. List the cities and states in which the authors live. See Figure 4.8 for the result.

    SELECT city, state
  FROM authors;

    Figure 4.8 Result of Listing 4.8. This result contains a duplicate row for San Francisco, California.

    city            state
--------------- -----
Bronx           NY
Boulder         CO
New York        NY
Palo Alto       CA
San Francisco CA
San Francisco CA
Sarasota        FL

    Listing 4.9. List the distinct cities and states in which the authors live. See Figure 4.9 for the result.

    SELECT DISTINCT city, state
  FROM authors;

    Figure 4.9 Result of Listing 4.9. It’s the city–state combination that’s considered to be unique, not the value in any single column.

    city            state
--------------- -----
Bronx           NY
Boulder         CO
New York        NY
Palo Alto       CA
San Francisco CA
Sarasota        FL
  • Although nulls never equal each other because their values are unknown, DISTINCT considers all nulls to be duplicates of each other. SELECT DISTINCT returns only one null in a result, regardless of how many nulls it encounters; see “Nulls” in Chapter 3.

  • The SELECT statement syntax includes the optional ALL keyword. You rarely see ALL in practice because it denotes the default behavior: display all rows, including duplicates.

    SELECT columns FROM table;

    is equivalent to:

    SELECT ALL columns FROM table;

    The syntax diagram is:

    SELECT [ALL | DISTINCT] columns
  FROM table;
  • If a table has a properly defined primary key, SELECT DISTINCT * FROM table; and SELECT * FROM table; return identical results because all rows are unique.
  • See also “Aggregating Distinct Values with DISTINCT” in Chapter 6 and “Handling Duplicates” in Chapter 15.
  • For DISTINCT operations, the DBMS performs an internal sort to identify and remove duplicate rows. Sorting is computationally expensive—don’t use DISTINCT unless you have to do so.

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